Public domain
[p. 620]
The most distinctive feature of this system in North America is its content of coal in the central and eastern part of the United States. It includes the Pottsville conglomerate (Millstone grit) below, and the Coal Measures above.
The lowest formation of the system in the Appalachian region is generally sandstone or conglomerate, having different names in different regions. From its conglomeratic phase in the east, it grades into sandstone at the west, and with local conglomeratic phases, the sandstone persists over the interior. It has not been recognized in the western part of America. Over wide areas it is unconformable on the Mississippian system, as already noted, and in places it appears to be a true basal conglomerate. In some places it is made up partly of cherts derived from the Mississippian limestone, showing that the latter had undergone prolonged decay before the deposition of the conglomerate. In other places the pebbles of the conglomerate seem not to have had a local origin. This argues for wide-spread emergence before the epoch of the Millstone grit, for on land, streams shift materials great distances. Locally as in Illinois, the formation is oil-bearing.
At various points in the east the formation contains thin beds of coal showing the local beginnings of the conditions which existed later over wide areas, and in the southern Appalachians it is ao important source of coal.
The formation varies in thickness from a maximum of some 1,500 feet in the Appalachians, to less than 100 feet in some parts of [p. 621] western Pennsylvania. The unequal thicknesses, even where the formation has not suffered from erosion, are due partly to the unevenness of the eroded surface on which it rests, partly to unequal rates of ’ sedimentation, and partly to unequal duration of the time of sedimentation in different regions. The formation is usually so firmly indurated that the outcrops of its tilted beds have become ridges.
Above the Pottsville conglomerate and its equivalents in the central and eastern parts of the continent, lie the formations known collectively as the Coal Measures. They consist of a succession of alternating beds of shale, sandstone, conglomerate, limestone, coal, and iron ore. The succession differs greatly in different regions, but shale and sandstone perhaps recur more frequently than any other members of the series, and in thicker beds. Both the coal and some of the iron ore are in layers interstratified with the other members of the series, and are to be looked upon as strata of rock. Important as the coal and iron ore are from an economic point of view, they make up but a small part of the Coal Measures. Although there are many beds of coal in some regions, and although some of them have great thickness (40 to 50 feet), the proportion of coal in the Coal Measures is rarely so much as 1 : 40, and that of iron ore is much less. The classification of the Pennsylvanian and Permian systems of the east now in common use is as follows:[1]
Present | Old | |
---|---|---|
Permian | Dunkard formation (or series) | = Upper Barren Coal Measures |
Pennsylvanian | 4. Monongahela “ “ “ 3. Conemaugh “ “ “ 2. Allegheny “ “ “ 1. Pottsville “ “ “ |
= Upper Productive Coal Measures = Lower Barren Coal Measures = Lower Productive Coal Measures |
A twofold division is common farther west. Thus in Iowa the lower division is called the Des Moines, and the upper, the Missourian.[2] [p. 622]
[p. 623]
In other regions, as in Ohio, a fourfold division was formerly made, as shown in the table above, but the uppermost is now generally referred to the Permian.
** The distribution of the unburied part of the Pennsylvanian system is shown in Fig. 435; also the areas where the system is believed to exist, though concealed, and where it is thought to have been removed by erosion. The system is probably concealed in areas to the south of those shown on the map, and its equivalentdoubtless underlies much of the sea.
The surface distribution of the system in the eastern part of the continent is in some ways in sharp contrast with the surface distribution of older systems. The commonest position for the outcrops of the preceding Paleozoic systems severally is around the outcrops of older systems. But the outcrops of the Pennsylvanian exhibit no tendency to a similar concentric distribution. Rather do they seem to cover areas between the outcrops of older formations. Thus in Michigan, the Pennsylvanian strata occupy an area completely surrounded by older formations.
This difference in surface distribution does not betoken any new principle in the distribution of the system. The Ordovician formations come to the surface, among other places, in New York, Ohio, Wisconsin, Missouri, and the Black Hills. Beneath the surface, the beds outcropping in these several localities are believed to be continuous, though concealed by younger formations. It will be remembered that most of the eastern interior, and perhaps most of the west as well, became land at the close of the Ordovician period. Had it never been submerged again, the Ordovician system would not have been covered, and its outcrops would now have appeared at the surface in broad areas between the outcrops of older formations; that is, its outcrops would have corresponded, in principle, with the surface distribution of the Pennsylvanian. After the deposition of the Pennsylvanian system, the surface where it is now exposed was elevated, relatively, and except for the glacial drift, was either never deeply covered by later deposits, or the overlying formations have been removed. Some of the separate [p. 624] areas of the system shown in Fig. 435 have probably been isolate. I by erosion since their deposition.
The Pennsylvanian system does not contain coal in workable quantity everywhere, though coal is widely distributed as far west as the 96th or 97th meridian in Oklahoma, and nearly to the 100th meridian in Texas. The productive coal areas of the system in the United States are five in number. These are as follows[3]:
(1) The anthracite field, which is confined to eastern Pennsylvania, and contains an area of 484 square miles. It includes several elongate, nearly parallel, synclinal basins, the longer a of which have a northeast-southwest direction (Figs. 436 and 4.>7). From the adjacent anticlines, and from the neighboring shallower synclines, the coal-bearing beds have been removed by erosion. The strata of this field may once have been continuous with those of the next.
[p. 625]
(2) The Appalachian field, which extends from the northern border of Pennsylvania to central Alabama, a distance of about 850 miles (Fig. 438), embraces an area of about 70,000 square miles, of which about 75 per cent contains workable coal. Speaking in general terms, the western edge of the sharply folded Appalachian belt forms the eastern edge of the Appalachian coal-field. With few exceptions, the strata of this field are gently undulating or horizontal. Few beds of coal are known to have great extent, but the Pittsburg bed in the Monongahela series, and the Sewanee bed in the Pottsville, seem to be continuous over areas of several thousand square miles.
(3) The Northern Interior field, confined to the southern peninsula of Michigan, covers an area of about 11,000 square miles. The strata of this field appear to dip gently toward the center of the basin (i. e., 1 < war* 1 the center of the Lower Peninsula). The formations of this basin were probably never connected with those of the other coal-fields.
[p. 626]
(4) The Eastern Interior field covers an area of about 58,000 square miles in Indiana, Illinois, and Kentucky (Fig. 438), and about 55 per cent of it is productive. This field is set off from the Appalachian field on the east, and from the Western Interior field on the west, by broad low anticlines from which the Coal Measures, if ever present, have been eroded. It is probable that this field was once connected with the next, and perhaps with the Appalachian field.
(5) The Western Interior and Southwestern fields constitute a nearly continuous area of Coal Measures formations, stretching from northern Iowa to central Texas, a distance of 800 miles, and covering an area of 94,000 square miles. On the east this field is limited by the broad low anticline which borders the Eastern Interior field on the west. It is limited on the west by the overlap of younger formations. Except in Arkansas and Oklahoma, where the strata are folded, the beds the Coal Measures of this area are esc tially horizontal.
East of the Appalachians. Tht Scotian-New Brunswick coal-field ties on either side of the Bay of Fundy and contains an area estimated at about 18,000 square miles. The coal is bituminous [p. 627] and of good quality. The system here attains a thickness of 13,000 feet.
In the vicinity of Narragansett Bay,[4] the Carboniferous system has great thickness, and often rests on beds of Cambrian age. Coal occurs here, but it is too highly anthracitic (or graphitic) to burn readily. The beds are much deformed and are associated with igneous rocks. Carboniferous rocks of undetermined extent occur at other points in New England, where they are partly igneous (Fig. 441) or meta-igneous, and partly meta-sedimentary. They are so completely metamorphic in some places as to make the determination of their age and relations difficult and uncertain. Beds of this age have recently been recognized in the Piedmont of Alabama.[5]
Distribution west of the Great Plains. The Pennsylvanian system is wide-spread west of the Great Plains, and probably underlies the Plains themselves. With rare exceptions, the western beds, largely of limestone and sandstone, are coal-less. The coal-less phase of the system, the whole earth considered, is far more widespread than the coal-bearing. The abundant coal of the west belongs to later systems.
[p. 628]
The Mississippian and Pennsylvania!! systems of the west have commonly been grouped together under the name Carboniferous. Most of the formations of both systems are marine, but sedimentation was somewhat generally interrupted at the close of the Mississippian period (p. 602). The formations of the Pennsylvanian period are not in general so wide-spread as those of the Mississippian, in this part of the continent.
In some parts of the west, the Carboniferous system includes formations which resemble the “Red Beds” of the next (Permian) system. This is the case, for example, in the southern part of the Rocky Mountain region, and in the plains adjacent, and here the separation of the Pennsylvanian system from the Permian is not very distinct or has not been carefully worked out.
The Carboniferous system of the west includes all sorts of sedimentary rocks, among which are considerable thicknesses of limestone. They are exposed at many points (Fig. and their existence over wide areas where they are now covered by later deposits is certain. The system is, however, not continous. Numerous islands of older rock probably maintained themselves throughout the period, and a large area of land existed throughout the Paleozoic era in western Nevada (west of long. 117°), and had an unknown extension north and south.
Many of the outcrops of the system in the west are about areas of older rock, and it is not always possible to tell whether these older formations were islands in the Carboniferous seas, or whether they once had overlying formations, now removed.
[p. 629]
Figs. 442 to 444 show the positions and relations of the Mississippian and Pennsylvanian systems at various points in the west. In most cases the sections are from regions where the strata have been much disturbed by folding, faulting, and the irruption of igneous rock.
North of the United States, Carboniferous strata (largely Mississippian) outcrop on the west side of the northward continuation of the Great Plains, and the strata here are probably continuous beneath younger beds which occupy the surface between them, and they are probably also continuous to the southward with the contemporaneous formations of the United States. Strata of the same age are found on both sides of the Gold Range of British Columbia. West of this range, much volcanic rock, the greater part of which was extruded before the close of the period (p. 601), appears in the system. The system is continued northward into Alaska,[6] where it is less wide-spread than the Mississippian, so far as present knowledge goes. In the Arctic lands of America, the Mississippian and Pennsylvanian are not differentiated, but one or both are somewhat wide-spread.
[p. 630]
Thickness. The thickness of the Pennsylvanian system has a wide range, but like all preceding systems of the Paleozoic, it is especially thick (4,000 to 5,000 feet) in the Appalachian Mountains. In the interior, the corresponding formations rarely much exceed 1,000 feet; but in Arkansas, the Coal Measures have been assigned the remarkable thickness of more than 18,000 feet, from which it is inferred that there must have been land close at hand capable of supplying sediments in great quantity. This was probably the axis of the Ouachita uplift. In Texas, the thickness of the system ranges up to 5,000 feet, and in the west, the maximum thicknesses exceed all those mentioned above, except that of Arkansas.
The general conditions under which sandstone, shale, and limestone originate have already been outlined, but there has been no occasion heretofore to consider the formation of coal. From its economic importance, this sort of rock has been studied with more care than most others, and geologists are agreed, in a general way at least, as to its mode of origin.
Origin. There is no doubt that coal is of vegetable origin. Except by the accumulation of vegetable matter, no way is known by which such beds of carbon could be brought into existence. Furthermore, the coal and its associated shales contain abundant remains of plants, sometimes even recognizable tree-trunks in the form of coal, and microscopic study has revealed the fact that the coal itself is often but a mass of altered, though still recognizable vegetable tissues. Concerning the exact manner in which the bedfl of vegetable matter accumulated, and concerning the conditions under which it was converted into the various sorts of coal, there is some difference of opinion.
Much of the coal is essentially pure, containing little ni.it t . any sort which was not in the plants which gave origin to it. Purity does not mean freedom from ash, since mineral matter, which on [p. 631] combustion becomes ash, is present in all plants.[7] Along with the large amount of coal which is pure, or nearly so, there is much which contains some admixture of earthy matter. Where the admixture of earthy matter is small, the coal may still be used; but from poor coal of this sort, there are all gradations into carbonaceous shale. Black shales are commonly associated with coal-beds.
The purity of some coal-beds over great areas warrants the conclusion that they were made of vegetation which grew where the coal is. The character of the vegetation of the coal shows that it grew on land or in swamps. Had it been washed down from its place of growth to the situations where the coal is, it should have been mixed with earthy sediment, and the product, after the necessary changes in the vegetable matter, would have been very unlike the purer coal-beds. Furthermore, the nearly uniform thickness of many of the coal-beds over great areas, sometimes many thousand square miles, constitutes a strong objection to the hypothesis that it was drifted together by any process whatsoever.
Some of the facts which support the theory that the vegetation grew where the coal-beds are, may be noted. Thus (1) beneath each coal-bed there is, as a rule, a layer of clay with roots (or root marks) in the position of growth. The clay seems to have been the soil in which the coal vegetation was rooted. (2) In association with the coal-beds, the stumps of trees are sometimes found still standing as they grew (Fig. 445). (3) In the coal-beds, or in the associated layers of shale, imprints of the fronds of ferns or fern-like plants are found. They are often so numerous and so perfect as to indicate that they were buried where they fell, without being drifted by moving waters from one place to another. (4) The layer of rock next overlying a coal-bed often contains abundant remains of vegetation, especially in its lower part, as if the conditions which brought about its deposition resulted in the destruction of the forest growth which had preceded. In such situations, trunks of trees 50 and 60 feet long, and 2 or 3 feet in diameter, are sometimes found. (5) The vegetable matter in and about coal-beds is made up of the trunks, small stems, leaves, and fruits of the various [p. 632] plants concerned, intermingled in such manner and proportions as to indicate that the vegetation grew where the coal now is. If the vegetation had been drifted together, these various constituents would hardly have been left commingled as they are. But while it is confidently believed that most of the workable coal represents the growth of vegetation in situ, it is not to be understood that coal was never formed from vegetation which drifted together.
In the formation of a coal-bed, three things are to be accounted for: (1) The conditions under which the necessary bodies of vegetation accumulated, often essentially free from the admixture of sediment; (2) how the vegetation was kept from decay; and (3) how it was changed into coal.
The accumulation of organic matter. Large marshes, or marshes in low surroundings, are the only places where vegetable matter is now accumulating in quantity, with little admixture of sediment. Thus in the marshes along some parts of the Atlantic coast (Fig. 446), there are great quantities of organic matter which, locally, is mixed with little sediment. In Dismal Swamp, [p. 633] the stems, branches, leaves, and fruits of the trees, shrubs, and herbs which grow there, have been long accumulating, and the great mass is nearly free from sediment. In various cypress and mangrove swamps, too, there are considerable thicknesses of vegetable matter nearly free from mud, etc. The multitude of marshes and peat-bogs in the United States and Canada are further illustrations of the accumulation of vegetable matter, sometimes mixed with abundant sediment and sometimes nearly free from it.
The vegetation in such situations need not be more luxuriant than on moist lands which are not swampy. On fertile prairies and in great forests the annual growth of vegetation is great; but since the leaves, fruits, twigs, and trunks decay as they fall, the larger part of their substance is returned to the atmosphere. In a moist region there is more growth (and therefore more death) of vegetation than in a dry one, and a better chance that decay will not keep pace with death. Decay is less rapid in a cool climate than in a hot one, so that in the former, there is more likely to be a residuum of partially decayed organic matter.
[p. 634]
Preservation of vegetable matter. In marshes, where the vegetation falls into water, it usually undergoes slow change different from decay suffered by vegetation which falls on dry land. The preserving influence of water is seen in many ways. Posts and piles set partly in water and partly above, decay just above the waterlevel, while the portions below remain sound. It is the partial preservation of organic matter in the water of marshes and very shallow lakes which converts them into peat-bogs, for the peat is nothing more than accumulated vegetable matter undergoing those changes to which vegetable matter in water is subject. Under favorable conditions, the peat of a bog may become very deep, as in the Dismal Swamp and elsewhere. In and about marshes and swamps, therefore, we find the conditions for the accumulation of considerable thicknesses of vegetable matter, sometimes nearly free from sediment, and at the same time the conditions which keep it from complete decay.
Conversion into coal. But while the vegetable matter is not destroyed, it is not preserved intact. The composition of woo* I and peat are illustrated by the following analyses (ash omitted), though neither wood nor peat has a constant composition.
Carbon | Hydrogen | Oxygen | Nitrogen | |
---|---|---|---|---|
Wood | 49.66 | 6.21 | 43.03 | 1.10 |
Peat | 59.50 | 5.50 | 33.00 | 2.00 |
The relative atomic proportions of carbon, hydrogen, and oxygen in cellulose are expressed by the formula C72H120O60. In the air, the carbon and the hydrogen of the wood unite with the oxygen of the air or the wood itself, forming carbon dioxide and water, the principal products of the decay of vegetation. Bui under water the atmospheric oxygen is largely excluded, and the elements of the wood are thought to unite with one another to a larger extent, while the oxygen of the air plays but a subordinate pari. ( >n< the common products of decay under such circumstances is ('II, (marsh-gas), which bubbles up from swamps and escapes into the atmosphere. The formation of this gas exhausts the hydrogen of the organic matter four times as rapidly as the carbon. It* the carbon of the wood unites with the oxygen of the wood, forming [p. 635] carbon dioxide, the oxygen is exhausted twice as rapidly as the carbon. If the hydrogen and the oxygen of the wood combine, the result is to increase the proportion of carbon remaining.
While the exact quantitative relations of the reactions which take place are not known, and are probably not constant, the following table [8] suggests certain changes which might take place, and the products which would remain at certain stages:
C72 H120 O60 (cellulose)— | 20 H20 8 CO2 2 CH4 |
= C62 H72 024 (peat). |
C62 H72 024 (peat) — | 6 H20 4 CO2 CH4 |
= C57 H56 O10 (brown coal). |
C57 H56 O10 (brown coal) — | 3 H20 CO2 2 CH4 |
= C54 H42 O5 (bituminous coal). |
C54 H42 05 (bituminous coal) — | 2 H20 CO2 5 CH4 |
=C48 H18 O (anthracite coal). |
From this table it will be seen that the process which converts vegetable matter into coal is characterized by progressive changes in the nature of the chemical decomposition. The elimination of hydrogen and oxygen in the form of water probably is the dominating chemical change in the production of peat from cellulose. Second in importance at this stage is the removal of oxygen in the form of carbon dioxide, while the liberation of methane (CH4) is of still less importance. As the alteration of the peaty material progresses through successive stages to anthracite coal, less and less water and carbon dioxide are given off, and there is a steady increase in the proportion of methane which is freed. Laboratory investigations have shown that while carbon dioxide may constitute an important part of the free gas held within the pores of some of the Cretaceous coals, the gas which escapes from the more advanced stages of Pennsylvanian anthracite coal is largely methane. The burial of the peat compresses it, and the physical change resulting is a part of the process of coal-making.
If the coal-beds represent Carboniferous swamps, as they are believed to, we have still to inquire into the conditions under [p. 636] which such extensive swamps existed, and to seek the explanation of their frequent recurrence (one for each coal-bed) in many regions.
The first condition for a swamp is lack of drainage, and the second a sufficient, but not an excessive amount of water. Enough to stop the growth of vegetation would be excessive, and too little to preserve it from prompt decay after its growth and death, would be insufficient.
Summary. In the course of the wide-spread movements which affected the eastern interior at the close of the Mississippian period, great areas appear to have emerged from the sea. Early in the Pennsylvanian period, considerable tracts which were not submerged stood so low as to be ill-drained, or undrained, and constituted marshes. Climatic conditions were such as to permit the growth of abundant vegetation in the marshes. On falling into the shallow water, the vegetable matter underwent changes of the nature suggested above. The marshes were thus converted into peat bogs. Some of the great coal-swamps probably came into existence along the sea-shores, and some in shallow basins or undrained areas remote from the sea, for fresh-water shells are found in association with some coal-beds, and marine fossils with others.
Each coal-bed represents the accumulated vegetable growth of a long period. It would appear that the growth and accumulation of vegetation was often brought to an end by subsidence which let the water (sea, or lake or aggrading stream) in over the marshes, drowning the plants, and burying the organic matter which had already accumulated under deposits of mud, sand, etc., which the submergence brought in its train. A second coal-bed in the same region points to the recurrence of swamp conditions, and moans either (a) that after submergence and burial of the organic matter slight emergence reproduced the conditions for bogs; or (b) that by sedimentation the sea or lake bottom where the first bog had been was built up to the water-level, restoring swamp conditions.
The number of coal-beds is often great. In Pennsylvania it frequently (but not everywhere) exceeds 20; in Alabama, 35 (not all workable) have been enumerated; in Nova Scotia, the Dumber, including some dirt-beds, is said to be about 80; but in the Missisippi [p. 637] basin west of the Appalachians, the number is often less than a dozen. In Illinois the number of workable beds is nine.[9]
Extent and relations of coal-beds. The wide-spread distribution of coal does not mean that any one marsh necessarily covered the whole of any one great coal-field. A few of the coal-beds, however, are of great extent. Thus the Pittsburg coal-bed is worked over an area of some 6,000 square miles[10] in western Pennsylvania, Ohio, and West Virginia, and has at least an equal extent where too poor to be generally worked. Many coal-beds, on the other hand, do not occupy great areas. From their thicker portions they thin out in all directions, often grading into black shale. From these facts it is inferred that within the general area of a coalfield there may have been elevations (islands) above the marsh level, interrupting the continuity of the swamps, and therefore the coal-beds.
Varieties of coal. The ways in which the different varieties of coal have arisen have never been satisfactorily determined. In general it is true that the anthracite coal occurs in mountainous regions, where the coal and other layers of rock with which it is associated have been subject to more or less dynamic action. Thus, in the mountains of eastern Pennsylvania (Fig. 436) the coal is mainly anthracite, while in the other coal-fields of the same age, where the strata are but slightly deformed, the coal is bituminous. In Arkansas, where the strata have been subject to some, but not to extreme dynamic action, coal is semi-anthracitic.[11] Where the metamorphism of the associated rock has been extensive, as in Rhode Island,[12] the coal has gone beyond the ant hracitic stage. Anthracite coal is also found in some places (not in the Coal Measures of the United States) in contact with dikes, in situations like those where other sorts of rock are metamorphosed.
These phenomena long ago suggested that anthracite is metamorphic [p. 638] coal, produced from bituminous coal by processes similar to some of those which metamorphose other sorts of rock. The fact that metamorphic coal is usually found in regions where erosion has exposed its beds (Fig. 437) has led to the conjecture that exposure of the coal may be a factor in the problem, the exposure favoring the escape of the volatile constituents, and so aiding in the transformation of bituminous coal to anthracite. Beds of bituminous coal are, however, often freely exposed. Both dynamic action, involving pressure and heat, and exposure would seem to be conditions favoring the development of anthracite, but it does not follow that these are the only factors in the problem, or that anthracite coal has never been produced in other ways. White has recently advanced the idea that deep-seated, horizontal thrust movements are the essential cause of devolatilization.[13]
There are several varieties of bituminous (soft) coal, some of which appear to depend on the nature and extent of the decay of the vegetable matter before its burial, and some on the degree to which the devolatilizing processes have been carried since burial. Recent studies[14] seem to indicate that the kind of vegetation entering into the make-up of the coal may have an important effect on the product. Thus some coal seems to be made up largely of algae, and such coal has rather distinctive qualities, if present interpretations are correct.
Other Products of Economic Value
Iron ore. The iron ore of the Coal Measures occurs in layers like the coal, or in the form of nodules which are often concentrated at a given horizon, forming a nearly continuous layer. The iron of the Coal Measures seems to have been largely deposited as a precipitate from the waters of inland and local basins while the other members of the system were being laid down. Dissolved by the land waters from the soil and rocks, it was brought to the marshes in some soluble form. In the marshes, it was precipitated either in the form of the carbonate or ferric oxide. Subsequent oxidation has changed some of the original carbonate into ferric oxide. The [p. 639] principal iron ores of the Pennsylvanian system occur in Pennsylvania and eastern Ohio.
The Pennsylvanian system yields oil and gas in some places, as in Oklahoma, Kansas, and Illinois.
General Considerations
Geographic conditions in the eastern interior. Returning for a moment to the system of which the coal-beds form an inconsiderable part, it is to be recalled that many of the clastic beds associated with the coal were laid down in fresh water, while other parts of the system were deposited in the sea. It follows that marine, lacustrine, and marsh conditions alternated with one another, and perhaps with land conditions.
The succession of Pennsylvanian beds in southwestern Pennsylvania (Fig. 439) illustrates the great series of changes which took place in sedimentation in the course of the period. The cause of the variation was probably geographic, but it is not to be inferred that geographic changes were more frequent at this time than during other periods. Their record is conspicuous because of the coal; or, in other words, because the land was near sea-level, so that extensive submergence and emergence resulted from slight changes of relative level of land and sea. It should be remembered that equally frequent and equally extensive movements, or that equivalent degradation and aggradation, would leave no such record of themselves, if the surfaces concerned were far above or far below sea-level. It was oscillation just above and just below water-level which allowed the record to be so clearly preserved. How far the oscillations were due to warpings of the land, and how far to changes in the level of the sea, cannot now be determined; but when we recall that the ocean-level must respond to every deformation which affects its bottom (unless compensated by an equivalent opposite movement), and to every stage of filling,[15] it does not seem strange that its level is in a nearly perpetual state of change.
In general, it may be said that the movements of the crust which have been of most importance, from the point of view of continental or biological evolution, are not those which have [p. 640] affected high land or deep sea bottom, but those which have converted sea bottom into land, or land into sea bottom. Such changes are most likely to have taken place where land was low, or water shallow. From the point of view of geology, therefore, the critical level of crust al oscillation is the level of the sea.
Duration of the period. So uncertain is our knowledge of the duration of geological time that all sorts of data which can be made to throw light on the subject are of interest, even though they do not lead to trustworthy numerical conclusions. Under favorable conditions, a foot of peat may accumulate in ten years or even less; but the usual rate is probably much slower. Peat bogs are now in existence in which the depth of the accumulated organic matter is 40 or 50 feet, but the length of time involved is not known. A vigorous growth of vegetation has been estimated to yield annually about one ton of dried vegetable matter per acre, or 640 tons per square mile. If this annual growth of vegetable matter were all preserved for 1,000 years, and compressed until its specific gravity was 1.4 (about the average for coal) it would form a layer about seven inches thick. But a large part of the vegetable matter, even in peat bogs, escapes as gas (CO2, CH4, etc.), in the making of coal. It has been estimated that four-fifths of it disappears in this way. If this is true, the seven-inch layer would be reduced to less than one and one-half inches, and a layer one foot in thickness would require between 8,000 and 9,000 years. The aggregate thickness of coal is frequently as much as 100 feet, and sometimes as much as 250 feet. At the above rate of accumulation, periods ranging from nearly 1,000,000 to nearly 2,500,000 years would be needed for the accumulation of such thicknesses of coal. It should be borne in mind, however, that much depends on the rate of growth of ( Carboniferous vegetation, which is not known.
On the other hand, these figures refer to the coal only, not to the Coal Measures. The greater part of the Coal Measures is made up of shale and sandstone, and of these formations there are thousands of feet, even where the sediments were largely fine and their accumulation therefore probably slow. It would hardly seem unreasonable to conjecture that their deposition may have consumed an amount of time equal to or or even greater than that [p. 641] demanded by the coal. Doubling the above figures, we get something like 2,000,000 and 5,000,000 years respectively, figures which must be taken to mean nothing more than that the best data now at hand indicate that the Pennsylvanian period was very long.
Close of the period. After the long period of oscillation above and below the critical level recorded by the Coal Measures, the interior east of the Mississippi was brought above the level of the sea, not to sink again beneath it during the Paleozoic era, and some of it at no later time. This emergence marks at once the close of the Carboniferous, and the inauguration of the Permian period. It is also probable that the deform ative movements which were to develop the Appalachian Mountains began at this time. There were notable changes also in the western half of the continent, for the Permian system is much less wide-spread than the Carboniferous. Where the Permian occurs, its constitution and its fossils indicate not only different relations of land and water, but different conditions of erosion, and the absence of the sea from some areas where deposition was in progress.
Europe
As in America, the oldest formation of the Upper Carboniferous in Europe is often a conglomerate and sandstone formation, the Millstone grit, which in some parts of Britain attains a great thickness. The Coal Measures of western Europe, like those of eastern North America, consist principally of shales (and clays), with subordinate amounts of sandstone and limestone. Associated with these commoner sorts of rock, there are beds of coal and clay-ironstone, both of which occupy positions corresponding, in all essential respects, with those of similar formations in eastern North America. There is workable coal in Great Britain, Ireland, Belgium, France, Spain, Germany, Austria, and Russia, but the total area of productive coal in Europe is much less than in America. The number of coal seams is large in many places. Thus in Westphalia the number of workable beds is said to be 90. The aggregate (maximum) thickness of the coal in Lancashire is 150 feet, and in Westphalia, [p. 642] 274 feet. Here, as elsewhere, beds of marine origin alternate with those which were deposited on land, in marshes, etc.
In Russia, as already noted, the Lower Carboniferous contains most of the coal, while the Upper is made up chiefly of limestone, though in southern Russia (Donetz coal-field) there is coal in the Upper division. The Upper Carboniferous limestone of Russia (Fusulina limestone) is represented by similar formations in southern Europe. The faunas of the marine part of the system in Europe have much likeness to those of western North America, suggesting that marine life was able to pass between these continents, via northern Asia.
The upper part of the Upper Carboniferous system of Europe at various points and at various horizons contains bowlders, sometimes of large size, and beds of breccia or conglomerate of subangular fragments. The bowlders are of granite, gneiss, schist, quartzite, etc., and occur both singly and in groups. They have often been thought to represent deposits made by icebergs, and so to point to the existence of glaciers; but this interpretation has not been established. Other interpretations, such as the transportation of the bowlders by uprooted and floating trees, are tenable.
North of the European continent, the Carboniferous formations (Fusulina limestone, Coral limestone, etc.) are represented in some of the Arctic islands (Spitzbergen, Nova Zemla, Bear Island).
Igneous rocks and crustal disturbances. Igneous rocks are associated with the Upper Carboniferous formations of sedimentary origin in western Europe. Their extrusion seems to have been an accompaniment of the crustal disturbances which began in middle and western Europe at the close of the preceding period, as already noted, and continued through the Permian.
Other continents. The Upper Carboniferous of Asia is represented by both marine and non-marine formations. Among the former is a thick limestone, wide-spread in southern China [16] and the Malay Peninsula, overlying the Devonian conformably. The nonmarine phase, with numerous beds of coal, is found in Asia Minor, on the east side of the Middle Urals, and in northern and eastern China, reaching to northern Tibet on the one side, and to Mongolia [p. 643] on the other. The Carboniferous of some parts of China has been reported to contain coal-beds of great thickness. The Carboniferous system is also present in India.
The Carboniferous formations of northern Africa correspond in a general way with those of southern Europe. They are generally of marine origin, so far as now known, and without coal, but in southeastern Africa, a coal basin has been reported in Zambesi.[17]
The Carboniferous (Permo-Carboniferous) system is well developed in Australia where it attains a thickness of 11,000 feet or more. It is partly marine and partly non-marine, and contains coal. The system is remarkable because of its singular conglomerates, some of which are of glacial origin. These will be referred to again in connection with the Permian.
In South America, rocks of Late (Upper) Carboniferous age are somewhat widely distributed, though less so than those of the Devonian period. The system has wide distribution in the lower part of the basin of the Amazon, where it rests on older formations unconformably, and is not generally coal-bearing, but is so in places. In southern Brazil the system contains much coal.[18]
With the opening of the Carboniferous period the supreme biological interest shifts from the sea to the land, and centers in the vegetation and in the amphibians.
Plant life was very abundant in the Pennsylvanian period, and the record of it is unusually full and perfect. This completeness has doubtless given the flora of this period an undue prominence over those which preceded ifc and succeeded; yet it was really a great period in the history of plant life. Angiosperms, the dominant plants to-day, had not yet appeared. Gymnosperms including the so-called “ seed-ferns,” held an important place. Pteridophytes probably made their greatest display at this time. All the [p. 644] great divisions of this group were present, and all of them were nearly or quite at their climax. An attempt is made to represent their general aspect in Fig. 447. Of lower plants, little is known.
The most rapid evolution of floras was perhaps in the Pottsville epoch. Half the genera of that epoch scarcely survived it, and few of them lived after the Allegheny epoch. This was pre-eminently the stage of Cycadofilices, Sphenopteris, Neuropteris and Alethopteris, and of the great lycopod types. The early Pennsylvania floras were widely distributed. Thus three floras in Asia Minor may be correlated severally with three floras from the Pottsville series. The place of origin of the early Pennsylvania floras is not known with certainty, but present evidence points to the land of western Europe and eastern North America, connected by an Arctic land bridge.
[p. 645]
The late Pennsylvania floras are not sharply separated from the early ones on our continent, but the distinction is much greater in Europe. The later Pennsylvanian floras indicate, on the whole, less uniformity of climate than the earlier.
The Filicales. Fern-like leaves surpass all other plant fossils in number, but it is now known that most of them belonged to seed [p. 646] bearing plants, though ferns were probably present. The ferns are a strangely persistent type. Species still live which, so far as outer form is concerned, might be referred to Carboniferous genera; yet under this general similarity of form, there have been notable changes of structure and function. They seem to have been, even at this early time, thoroughly differentiated from other plants.
The Equisetales (calamites, horsetails). These plants, represented now by a single genus (Equisetum) , were an important member of the Carboniferous flora. The calamites were not only larger, but more highly organized than the modern representatives of this group. The largest tropical forms of to-day have slender stems 30 or 40 feet long, whereas the Carboniferous calamites reached a foot or two in diameter and probably 60 to 90 feet in height. They had hollow stems, or a core of pith only, and casts of the interior are common fossils. Branches from the main trunk were comparatively few, and in whorls. The leaves also were in whorls (Fig. 447) and dwarfed, though not so much so as in the modern type, in which the leaves have almost disappeared. The structure of the leaves was of the type adapted to dry weather (xerophytic) as in the pine and in many desert plants, and also, contradictorily enough, as in some undrained swamp plants. The root structure was of the type commonly found under water or in wet mud, and the calamites probably frequented swamps and lowlands. The calamites were probably associated in thickets and jungles of cane-brake or bamboo type. Their history may run far back, as they were well differentiated-in the Devonian. Their ancestry is uncertain, but the next group throws light on their relations.
The Sphenophyllales. Recent studies have shown that the graceful, slender plants with whorled leaves, referred to the genus Sphenophyllum (c,Fig. 449), and formerly classed as calamites, should be made a class by themselves. Their importance lies chiefly in the fact that while they have certain calamarian features, they have others possessed by the lycopods. This is interpreted to mean that these two groups (calamarians and lycopods, p. 944) were united with the Sphenophyllales in a common ancestral form.[19] The stems were long, slender, and apparently weak, and a climbing [p. 647] habit has been inferred. The leaf structure suggests a shady habitat, perhaps one of undergrowth. The class was represented in the Devonian, had its climax in the middle Pennsylvanian, and continued into the Permian and possibly later.
The Lycopodiales. This was the master group of the Coal flora, constituting trees of large size and attaining to the highest organization reached by the pteridophytes. From this high estate, they have since fallen to prostrate or weakly ascending plants of mosslike aspect (club mosses and ground pines) . The chief genera were Lepidodendron (Fig. 447) and Sigillaria, of which the former was the earlier and simpler type. Both take their names from the leafscars of leaf-cushions (lepidos= scale, sigilia=seal) which the trunks [p. 648] retained. In the lepidodendrons the scars are arranged spirally (Fig. 450); in the sigillarians, vertically (Fig. 451).
The trunks of lepidodendrons were tall, some having been found 100 feet in length. They were erect and branched dichotomously at a great height. The leaves were linear or needle-shaped, ranging up to six or seven inches in length, and set densely on the branches. Some of them were heterosporous, a characteristic pointing in the direction of seeds, but it is not known that seed-producing plants sprang from them. One form of the fruit was distinctly winged, and other forms showed adaptation to transportation by wind. More than 100 species of lepidodendrons have been described. They seem to have reached their climax early in the period, and nearly all had disappeared by its close.
The sigillarians differed from the lepidodendrons in being mostly without branches. They were perhaps the largest of the Carboniferous trees, their trunks reaching six feet in diameter, and 100 feet or more in height. As in lepidodendrons, the stems were densely clothed with erect, rigid, linear leaves. Like the lepidodendrons, they had a thick cork layer, rarely equalled in modern trees, except in the cork-oak and its allies. The sigillarians exceeded the lepidodendrons in abundance before the close of the period, but were on the wane at its close. Perhaps this rather sudden decline, followed by early extinction, was connected with the changes of climate indicated by the Permian glaciation, though this cannot be affirmed.
[p. 649]
The Cycadofilicales. It has been determined recently that many of the fern-like forms possessed structural features which combine the characteristics of ferns and gymnosperms; but since they were seed-bearing, they are classed with the latter. The seeds were borne on leaves, in positions similar to those of the “fruit dots” of ferns. One of the best-known forms is illustrated by Fig. 452, which exhibits the spiny stems and leaves, the highly dissected foliage, the adventitious roots, and shows the general aspect of the type. The structure of their leaves “is altogether comparable to that of a fairly coriaceous fern-leaflet at the present day, and indicates that the conditions to which the structure was adapted could not have been fundamentally different from those which prevail in our own epoch.” [20] The limits of the group are not yet known, nor are its connections with the true ferns. Initial forms have been identified from the Devonian and Lower Carboniferous, and a few are found in the Permian; but the group is essentially Pennsylvanian.
[p. 650]
The Cordaitales. This was a remarkable family (now extinct) of gymnosperms (p. 944), having alliances with the "seed-ferns,‘’ conifers, and ginkgos, and yet many distinctive features of its own. The Cordaites were tall, rather slender trees, reaching two feet or more in diameter, and 90 feet or more in height. The wood was of the coniferous type, covered, as in so many other plants of the period, by a thick bark. The trunks had a large pith. The leaves were parallel veined, suggestive of monocotyls of the yucca type, and sometimes attained a length of six feet and a width of six inches. They were preserved in great abundance, and make up a large part of some beds of coal. The leaf-structure combined characters now possessed by certain conifers, with others possessed by certain cycads. In one form, the leaf had a distinctly fleshy character, as if adapted to xerophytic life.[21] The floral organs were peculiar to the family, and have been worked out with marvelous success, even the structure of the pollen having been determined. The inflorescence took the form of separate male and female catkins, arranged on slender stalks attached to the stem between the leaves. The seeds (Cardiocarpus) were of the cycadian rather than of the coniferous type, and were very abundant and sometimes winged, as if for wind transportation.
It is doubtful whether conifers existed in the Pennsylvania^ period, though they were probably represented in the Permian. The upland vegetation is not known, and it is not impossible thai conifers, a type especially suited to an upland habitat, prevailed there.
Cycadales have been commonly reported from the Carboniferous, but the evidence remains inconclusive, and the fossils concerned are probably Bennettitales, rather than true cycads.
The Coal flora of North America and that of Europe were strikingly similar, implying close geographic relations and like conditions. [p. 651] Nearly all the genera, and about one-third of the species, were identical.
Climatic Implications of the Coal-plants
What suggestions do the Coal-plants give relative to the atmospheric conditions under which they grew? Two partly antagonistic views relative to these conditions have been held. The older one regards the thick deposits of coal as evidence of a very luxuriant growth of vegetation, which in turn has been thought to imply a warm, moist atmosphere, heavily charged with carbon dioxide. The great size of many of the trees, the succulent nature of many of the plants, and the abundance of aerial roots, are appealed to as evidence of mildness of climate, while the absence of rings, and, above all, the great geographic distribution of the floras in relative unity throughout very diverse latitudes, point strongly to equability, an equability which was more pronounced early in the period than toward its close. This view was formerly about the only one, and still predominates, and it is clear that it has much support.
The alternative view which has grown up in recent years postulates less warmth and moisture, and more diversity; in other words, a somewhat nearer approach to the present conditions. It assumes, however, a somewhat higher percentage of carbon dioxide than now, and a climate milder and more uniform than that of to-day. The basis of this view is found in the following considerations: (1). Great thicknesses of coal do not necessarily imply rapid accumulation, any more than great thicknesses of limestone do. Given favorable conditions of preservation, a slow growth will produce great thicknesses. (2). At present the accumulation of peat, the nearest analogue of coal formation, is most favored in cool climates, and is taking place chiefly in high latitudes. (3) . The dominant plants of the coal flora had narrow leaves with their breathing pores confined to deep furrows on the under side, devices common to plants of dry regions. (4). The trees had unusually thick corky bark, as though protection from external conditions was needed.
The thickness of the bark, and the form and structure of the [p. 652] leaves, give a distinctly xerophytic aspect to the overgrowth made up of lepidodendrons, sigillarias, calamites, and cordaites. This is not the case with the undergrowth, but this would not be expected of shaded plants. The force of the inference from the xerophytic aspect of the overgrowth is, however, much weakened by the fact that the vegetation of undrained swamps and bogs assumes many of these xerophytic features. It is clear that a more critical study of the problem is necessary before a final conclusion concerning the climate of the period is reached.
Amphibians, insects, spiders, scorpions, and myriapods, lived on the land at this time. The amphibians are perhaps of chief interest, for they were the first of the great line of land vertebrates, the ruling dynasty from that day to this. So far as the evolution of air-breathing vertebrates is concerned, this is one of the most important periods in geological history.[22]
The rise of the amphibians. Tracks attributed to amphibians are found in the Devonian and the Mississippian, but in neither of these systems have any bones of these . animals been found in America, and only imperfect ones in Europe. When the fossils of amphibians first appear in abundance, in the later Coal Measures, they were already so differentiated as to imply a long antecedent existence. Most of them were rather primitive in structure, but they were genuine amphibians, not transition forms. In the < of many of them, fossils representing all stages of growth have been found, showing that the young had gills, and that the gills were lost later in life. All of them seem to have had elongate forms of salamandrine aspect, and their heads were well roofed over by the bony plates of the skull. On account of this last feature they are called stegocephalians (roof-headed). Some of them have also been named labi/rinthodonts, from the intricate infolding of the dentine of their teeth. The group varied in length and strength of limb, in agility, ability to climb, etc. The elongation [p. 653] of their bodies involved a notable multiplication of the vertebra?, one form having no less than 150. In places, their tracks are so abundant as to imply great numbers of individuals, at least locally.
The predominant forms were branchiosaurs and microsaurs. In size and in habits, the branchiosaurs at least were comparable to the salamanders of to-day. The microsaurs, on the other hand, had made distinct advance, both toward higher types, and away from water, in and about which the branchiosaurs lived. Some of the microsaurs lost their dermal armor, and became fleet, like modern lizards. Before the close of the period, some of them were probably inhabitants of dry lands where fleetness, rather than protective armor, preserved them from their enemies. Differentiation went so far before the close of the period that some of them were limbless and snake-like, crawling reptiles in everything except certain technical details of their palates.
Two other types, Temnospondyli (African) and Stereospondyli, either persisted from unknown ancestors, or made their advent at this time. The temnospondylous branch, which reached its highest development in the Permian, is supposed by some paleontologists to be the ancestral line of all modern reptiles, and by others to be the ancestral stock from which mammals arose. The stereospondylous branch, which included the labyrinthodonts, is the only group of Pennsylvanian air-breathing vertebrates which left no descendants. [p. 654] The labyrinthodonts were doubtless the largest amphibia of the period, some of their skulls reaching a length of half a meter or more.
Not much is known of the food and life-habits of the amphibia, but from their teeth it is inferred that they were predaceous. In Nova Scotia, Dawson took thirteen skeletons representing different species of amphibians from a single sigillarian stump. Since land shells and myriapods are found in stumps with the amphibian skeletons, it has been inferred that some of the amphibians were climbers, and lived on mollusks, myriapods, and similar land life.
The amphibians of different continents were so similar as to suggest great freedom of communication and migration. This free intercontinental migration seems to have come to an end by the close of the Pennsylvanian period.
The marked development of insects.[23] Several hundred species of Carboniferous insects have been identified from the American Coal Measures, and a comparable number from European. They were still, for the most part, of rather primitive types, often uniting characters not now found in the same order. The orthopters (cockroaches, i, Fig. 454, locusts, crickets, etc.) were greatly in the lead, followed by the neuropters (represented by ancestral mayflies). These two orders include about 90 per cent of the known insects. Hemipters (bugs), which had appeared earlier, and possibly coleopters (beetles) were present, but no fossils of bees, butterflies, or moths have been found, and there is no probability that they existed, since the flowering vegetation on which they depend had not yet appeared. There is also no record of flies. The evolution of insects was therefore one-sided. Curious forms were developed within the orders which lived, and remarkable dimensions were attained, spreads of wing of a foot or more being reported.
Spiders and myriapods (Fig. 454) were plentiful. Scorpions (g) also were present, and several species of land snails (d and e ) have been identified. The air-breathing community had become [p. 655] large and diverse. The amount of carbon dioxide in the atmossphere could not have exceeded that compatible with this life.
Besides fresh-water plants, the life of land waters appears to have consisted of fishes, mollusks, crustaceans, probably of the larval forms of certain amphibious insects, and doubtless of many [p. 656] unknown forms. Aside from the developments of the fresh-water fish and of the amphibians, perhaps the most suggestive feature was the association of the arthropods with other forms of life.
Eurypterids (Fig. 455) were still in existence, and their relics are so intimately associated with beautifully preserved ferns, calamites, insects, spiders, and scorpions as to leave no reasonable doubt that they were fresh-water forms. There were also crustaceans resembling crayfish, and others of shrimp-like appearance.
Because of the approximation of great areas of the continent to sealevel, two phases of sea life had sufficient prevalence to be worthy of note. The first consisted of those forms that habitually occupied the thin edges of the sea, which, in the form of estuaries, lagoons, and shoals, crept in and out on the borders of the continent as the relations of land and sea oscillated, while the second embraced the life of the more open seas. This distinction had doubtless always existed, but it had not before reached equal importance. Life of the first phase was not usually well preserved, but in the coal regions of this period, it constitutes the better part of the record. In this phase, where sandy and muddy flats prevailed, pelecypods and gastropods, together with certain fishes, predominated, while in the more open seas the brachiopods, cephalopods, and the clear-water types were more plentiful.
It is difficult to tell which of the fishes should be regarded as marine, which as fresh water, and which as common to salt and [p. 657] fresh water. It seems clear that much the larger number of those found in the American Coal Measures lived in fresh water; whether also in salt water is uncertain. It is probable that the sharks armed with shell-crushing teeth were chiefly marine, and those with cutting teeth largely so. During the period, there was progress among the fishes in adaptation to swift movement, and in shapeliness of form.
The invertebrates. By comparing the pelecypods represented in Fig. 456, with earlier forms, it will be noted that this group had assumed a more modern aspect. The period was doubtless favorable to their advance, by presenting an extensive but shifting habitat that both invited and forced adaptive change. The gastropods, on the other hand show less departure from earlier types. They are still distinctly Paleozoic. Ancient and relatively modern types of cephalopods lived together, the former represented by straight, plain, small orthoceratites (z,Fig. 456) that might well belong to the earliest Paleozoic period, and the latter by closely coiled goniatites (zz) with curved sutures that might well be early Mesozoic types. The orthoceratites were about to take their final leave, and the goniatites were about to evolve into ammonites, the dominant type of the Mesozoic era.
The brachiopods held an important place, and their general facies was like that of the later Mississippian. Some species range not only through northern America and Eurasia, but into the Orient and Australasia. Crinoids were a smaller element of the fauna than might have been expected from their previous and subsequent history. There was a close relation between several American and Russian crinoids, implying intermigration. The cystoids and blastoids were gone. No starfishes have been recognized, though they were doubtless present, and sea-urchins were rare. Trilobites, which commanded foremost attention at the opening of the Paleozoic, are now almost at the point of disappearance. The last representative of the group had the chaste beauty of its early ancestors. Bryozoans were not uncommon, but the peculiar devices for support illustrated in Archimedes and Lyropora of the preceding period were abandoned. Protozoans were represented widely by a little foraminiferal shell (Fusulina secalicus, b,Fig. 456), which, had about the size and form of a grain of wheat. Their abundance gives character to the Fusulina limestone which occurs in America, Europe, and Asia. Corals were rare, as might be expected under the environmental conditions of the time.
[p. 658]
[p. 659]
Map work. No reference to map work has been made since that at the close of the chapter on the Ordovician, p. 535. Experience has shown that if the principles of stratigraphy as illustrated by the Cambrian system, are well developed as suggested on p. 506, further map work may be deferred to about this point. Many of the maps available for this work involve most of the Paleozoic systems of rock, and if map work on these systems is taken up at this point, it may be made to serve as a review of the history of the periods when these systems were deposited.
The following folios of the U. S. Geological Survey are among those especially serviceable in this connection: Arizona, Clifton, Globe; Arkansas- Missouri, Fayette ville; Arkansas-Oklahoma, Winslow; California, Colfax, Nevada City Special, Redding; Colorado, Needle Mountains, Walsenburg; Illinois-Indiana, Danville; Indiana-Illinois, Patoka; Kansas, Independence; Massachusetts, Holyoke; Missouri, Joplin; Montana, Livingston; New Jersey, Franklin Furnace; New York, New York City; Oklahoma, Atoka, Tishomingo; Pennsylvania, Ebensburg, Elkland-Tioga; Tennessee, Briceville, Kingston, Standingstone ; Tennessee-Georgia, Ringgold; Utah, Tintic; Virginia-Tennessee, Bristol; Virginia-West Virginia, Monterey; Washington, Snoqualmie; West Virginia, Charleston; Wisconsin, Lancaster-Mineral Point; Wyoming, Alladin, Cloud Peak-Fort McKinney, Sundance.
If a fuller list is desired, the following may be added: Arizona, Bisbee; Colorado, Ouray, Rico; Illinois, Chicago; Maine, Penobscot Bay, Rockland; New Jersey, Passaic; Oklahoma, Muscogee, Talequah; Tennessee, Chattanooga; Wisconsin, Milwaukee; Wyoming, Absaroka (Crandall Sheet), Bald MountainDayton, Yellowstone. If the full set of folios is available, folios even more suitable for special localities may be selected.
Prosser, Am. Jour. Sci., 4th series, Vol. XI, p. 191, 1901. For recent review of the Pennsylvanian system of the Appalachian region, see Stevenson, Bull. G. S. A., Vol. XVII. ™ S ↩︎
Geol. Surv. of Iowa. ↩︎
Hayes, Stoek, White, Campbell, Haseltine, Lane, Ashley, Bain, and TafT. 22d Ann. Rept., U. S. Geol. Surv., Pt, III, p. 15. ↩︎
Shaler, Woodworth, and Foerste, Geology of the Narragansett Basin, Mono. XXXIII, U. S. Geol. Surv. ↩︎
Smith, E. A., Underground Water Resources of Alabama, Geol. Surv. of Ala., 1907. ↩︎
Brooks, Professional Paper 45. ↩︎
Many of the modern allies of the coal-plants contain as much as five per cent of ash, and a few much more. ↩︎
Prepared by Rollin T. Chamberlin. ↩︎
Reports on coal have been published by all states containing coal, where there have been surveys. For local details see reports of Pennsylvania, Ohio, Kentucky, Tennessee, Alabama, Indiana, Illinois, Iowa, Missouri, Arkansas, Kansas, and Texas. ↩︎
White, West Virginia Geol. Surv., Vol… II, p. 166. ↩︎
Ann. Rept. Ark. Geol. Surv., 1888, Vol. III. ↩︎
Geology of the Narragansett Basin, Mono. XXXIII, U. S. Geol. Surv. ↩︎
David White. Economic Geology, Vol. III. ↩︎
Especially those of Mr. White. ↩︎
Salisbury, Jour. Geol., Vol. XIII, p. 469. ↩︎
Researches in China, Carnegie Institution, Willis, Vol. I. ↩︎
Kayser, Geologische Formationskunde, p. 207. ↩︎
White, I. C. Commissao de Estudos das Minas de Carvao de Pedra da Brazil, 1908. ↩︎
Reward, Fossil Plants, p. 413; Scott, Studies in Fossil Botany, p. 404. ↩︎
Scott, Studies in Fossil Botany, p. 326. ↩︎
Scott, op. cit., p. 425. ↩︎
Williston. The Faunal Relations of the Early Vertebrates, Jour. GeoL, 1909. ↩︎
Scudder, Bull. No. 71, U. S. Geol. Surv., 1891, and works there referred to. Brongniart, Researches pour servie a l’histoire des Insectes Fossiles dea Temps Primaires, 1894. Dawson, J. W., Synopsis of the air-breathing animals of the Paleozoic in Canada up to 1894. ↩︎