Public domain
[p. 507]
The general conformity [1] between the Cambrian and Ordovician systems shows that no considerable physical change took place in the relations of land and water in North America at the close of the Cambrian period. At the opening of the Ordovician, therefore, as at the close of the Cambrian, an epicontinental sea stood over much of the continent.
Sedimentation During the Ordovician Period
The conditions of sedimentation during the Ordovician period were somewhat different from those of the Cambrian. All the common processes of weathering were operative on such lands as still existed, wasting the rocks and preparing sediment for removal to the sea; but the small area of land within North America yielded but little sediment, and during much of the period the deposition of land-derived sediment was confined to littoral tracts. Farther from land the shells, skeletons, and other secretions of marine animals and plants were accumulating, making limestone. Since the land areas of the period were of various sizes, of various sorts of rock, and presumably of various heights, it is probable that conditions existed for the deposition of all sorts of clastic sediments about their borders, and for their deposition at very different rates. Sedimentation was doubtless more rapid near the larger and higher land masses than about the smaller and lower ones, and more rapid on that side of any land towards which the larger part of its drainage was directed.
The sedimentary formations of the Ordovician period are in [p. 508] keeping with these general principles. Adjacent to the broad, shallow arm of the ocean which covered the larger part of the Mississippi basin (Fig. 358) there appear to have been no sources of abundant sediments during most of the period. Along the western base of Appalachia, mud, sand, and gravel, washed down from the land, were being deposited. The coarser materials were left nearer the land, while the finer were carried farther out. Alternating beds of coarse and fine sediment indicate either (1) that the adjoining land was higher at some times than at others, or (2) that the climatic conditions or (3) the vegetal covering changed, or (4) that waves and currents varied in their effectiveness.
The sediments deposited at the same time in Newfoundland, in the northeastern parts of Canada, and in the Ottawa basin, were largely of limestone, indicating the absence of abundant debris from land in these regions. About the isolated land masses farther west, sand and mud, to become sandstone and shale later, were in process of accumulation; but the sources of material appropriate for such formations were not extensive, and the formations themselves are correspondingly limited. Conditions for the formation of limestone prevailed widely in the epicontinental sea. Plants and animals secreting calcium carbonate may have been no more abundant far from land than near it, but away from shore their shells, etc., were probably more abundant relative to the sediments derived from the land. The occasional variations from limestone to shale or sandstone in the interior of the continent show that physical conditions were not altogether constant.
But even during those intervals when the land was so low as not to yield abundant sediments, preparation was making for future formations of clastic rock. The formations of the land were undergoing decay, even though the products of decay were not removed. Under these conditions, thick mantles of residual earths accumulate, representing the excess of rock decay over transportation. During such periods of rock decay a large. amount of disintegrated material is made ready for removal when uplift of the land rejuvenates the streams.
The development of the Ordovician system meant the destruction of an equivalent body of older rock. The old material which [p. 509] entered into the new system, derived from all preceding formations so situated as to be exposed to erosion, was brought from the land by streams, worn from its shores by waves, or blown to the sea by winds; and where terrigenous sediments failed, or where they were relatively unimportant, the secretions of the animals and plants accumulated, giving rise to sedimentary rocks of organic origin. Even these had their ultimate source in the older formations, for the mineral matter extracted from the sea to make the shells had been dissolved from older formations during the process of their decay, and brought to the sea in solution, often by the same streams which brought the mud in suspension.
It is probable that the larger part of the ocean basins was continuously submerged during the Ordovician, as during earlier periods, and that in them the Ordovician strata overlie those of Cambrian age conformably. Though nothing can be known directly of the Ordovician system beneath the sea, it is important, in the conception of the system as a whole, to remember that it probably underlies most of the oceans as well as many of the younger formations of the land, and that its exposed margin is but a trivial fraction of its total area.
Sections of the Ordovician
The New York section. The Ordovician system of North America was first studied carefully in New York, and the section of that State is, in some measure, the standard to which others are referred. The system in New York is now divided as follows:
[p. 510]
Other sections. The classification of New York is not applical >le in detail to the system in other parts of the continent. In Wisconsin, Iowa, and Minnesota, for example, the formations commonly recognized, numbered in the order of age, are as follows:
It can hardly be affirmed that any one of these formations is the exact equivalent of any one in New York.
In the Appalachian Mountains[4] of Tennessee, a series of limestone or dolomite beds (Knox, Chickamauga, etc.), the lowest not distinctly marked off from the Cambrian below, is followed by a series of clastic beds (Sevier shale, Bays sandstone, etc.).[5] The exact relations of these formations to those of New York and to those of the upper Mississippi basin have not been determined, and since the strata between Tennessee and these localities are concealed for the most part, the relations must remain unknown, except in so far as the fossils may reveal them. The section of Tennessee does not correspond in details, with that of other parts of the Appalachian belt.
General conditions in the eastern part of the continent. It is worthy of note that in mid-Ordovician time, limestone was forming from New England on the east, to Georgian Bay on the northwest, to Oklahoma and Texas on the southwest, and Alabama on the south. Limestone was forming also in much of the west. At no previous epoch was there anything like such wide-spread deposition of limestone within the limits of our continent. The explanation of this condition of things has been suggested already (p. 508). It [p. 511] is perhaps equally worthy of note that in the latter part of the period, mud (now shale) was deposited over an almost equally extensive area. This may mean either that the lands were so elevated as to allow the streams to carry more sediment to the sea, or that conditions favored the transportation of mud farther from shore than formerly, or both. Associated with the Upper Ordovician shales, there are considerable bodies of limestone in some places, and of sandstone in others. All the Ordovician formations of the interior and the east bear within themselves evidence of shallow water origin.
Western sections. In the Great Plains, the Ordovician system appears at the surface but rarely, though it probably underlies the younger formations. West of the Great Plains, the system is present generally, and the sections are somewhat simpler than in the interior or the east. As farther east, limestone is a conspicuous part of the system here.
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[p. 513]
Igneous rocks. Igneous rocks of Ordovician age attain little importance in North America. Their general absence is in harmony with the quiet which characterized the period.
General Conditions and Relations of the Ordovician System
Position of beds. As originally deposited, the Ordivician beds probably dipped away from such lands as then existed. Thus on the south side of the land of northern Wisconsin (Fig. 382) , the Ordovician sediments must have dipped slightly to the south, and on the east and west sides, to the east and west respectively. The same relations held about every land area. Over great areas in the interior, this original and simple plan of stratigraphy has been ^ but little modified (Fig. 378).
In other regions, deformation of the strata has completely changed their original positions. Thus in the Appalachian Mountains, where the sediments were derived principally from the land to the east, and where the beds doubtless had a slight dip to the west at the time of deposition, they now dip in various directions and at various angles, as the result of folding. Faulting has complicated their structure still further (Figs. 379 and 380) . The strata are in similar positions in some parts of Arkansas (Fig. 381), Oklahoma, and the various mountain ranges of the west.
Condition of the formations. The sediments have undergone more or less alteration since their deposition. In some places the changes have been slight, and in others great. The larger part of the Ordovician sands are now in the condition of sandstone, the larger part of the muds in the condition of shale, and most of the limestone is still essentially non-met amorphic. But where dynamic action has been great and where the original position of the strata has been greatly changed, the changes in the rock have been greater.[6] Thus in the Taconic Mountains (southeastern New York and southwestern New England), the limestone is mainly in the condition of marble, the sandstone and quartzite have been largely changed to quartz schist, and the shales to slate and schist. Met amorphic rocks of Ordovician age are known also in some parts of the Piedmont plateau.
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[p. 515]
Thickness. The rocks of all systems vary greatly in thickness, and the Ordovician system is no exception. In the Appalachian Mountains it is to be measured by thousands of feet, while in the interior it is to be measured by hundreds instead. In Wisconsin and Iowa, where sedimentation seems to have been interrupted but little from the beginning of the period to its end, the aggregate thickness is rarely 700 feet.
Width and position of outcrops. In the interior, where the system is relatively thin, it sometimes appears at the surface in relatively wide belts or areas (Fig. 382) , while in the eastern mountains, where it is thick, it appears at the surface in a succession of narrow and parallel belts (p. 488).
Close of the Ordovician Period
The close of the period was marked by geographic changes of more importance than those at its beginning. The greatest change was the withdrawal of the epicontinental waters from a large part of North America, converting extensive stretches of shallow-sea bottom into land. The cause of this change may have been the sinking of the ocean bottoms and the drawing off of the epicontinental waters. The altitude of this new land must have been slight or its exposure brief, for it suffered little erosion before much of it was again submerged and covered by sediments of later age. It is indeed the wide-spread absence of the lower part of the Silurian system (p. 536), rather than a pronounced stratigraphic mconformity between it and the Ordovician, which indicates the [p. 516] extensive emergence of land in the interior at the close of the Ordovician period. Throughout much of the western part of the continent also, the land may have emerged at about this time, for the Silurian system is wanting, or has not been recognized, in many regions where the Ordovician is present.
Folding movements were less wide-spread. The most considerable was in the Taconic Mountains, where both the Cambrian and Ordovician systems were thick. Both were folded and lifted above the sea beneath which they had accumulated. The eroded remnants of the folds often show a complicated structure. The date of the folding is known, because Silurian formations overlie the Upper Ordovician unconformably about the borders of this mountain region. It is not to be inferred that all the mountain-making movements which have affected western New England occurred at this time. There had been folding earlier, in pre-Cambrian times, and there were later movements, as will be noted.
Between folding and the more gentle movements already noted there are all gradations. The “Cincinnati arch” is an example. This arch is a very low anticline with a general north-south course, extending through Cincinnati. The beginning of this arch may have been as early as mid-Ordovician.[7] Another similar arch[8] may have come into existence at about the same time in Arkansas and Oklahoma, corresponding in position with the mountain system commonly known as the Ouachita Uplift, of which it was perhaps the beginning. The strata of this region were notably folded at a much later time. In some other places, as for example in New Brunswick and Nova Scotia, there is unconformity between the Ordovician strata and those which overlie them, indicating an emergence after the deposition of the Ordovician formations.
The crustal movements referred to above have been mentioned as occurring at the close of the Ordovician. It would perhaps be more accurate to say that their beginning marks the beginning of [p. 517] the transition from the Ordovician period to the Silurian. The duration of the interval of transition was probably long.
Economic Products
In Ohio and eastern Indiana the Trenton formation yields much gas and oil.[9] Both these substances are believed to be products of the decay or distillation of organic matter which was included in the sediments at the time of their deposition. The oil is most abundant under low anticlines, where it occurs in the pores and openings of the rock, somewhat as ground-water does.
The Galena and Trenton formations in Wisconsin[10] and in the adjacent parts of Iowa and Illinois contain ores of lead and zinc, mainly in the form of sulphides and carbonates. Lead ores are also found in the Ordovician (or Cambro-Ordovician) formations of southeastern Missouri,[11] and lead and zinc ores in the southcentral part of the same state. In all these regions the ores occur (1) in cavities formed by solution, (2) as replacements of limestone, or (3) in crevices. In these positions, the ore was concentrated by ground-water. The metallic substances were doubtless derived from the limestone itself, which, at the time of its deposition, is thought to have contained trifling amounts of lead and zinc, derived from sea-water by organic deposition.
The Ordovician limestones of central Tennessee[12] locally yield calcium phosphate, valuable as a fertilizer. The workable deposits have. resulted from the leaching of phosphatic limestone, leaving the less soluble phosphate concentrated at the surface (Fig. 383). The Manganese ore of Arkansas had a similar origin. The metamorphic Ordovician limestones of New England and some parts of the Appalachian Mountains have been extensively used for marble.
Foreign Ordovician
The Ordovician formations appear at the surface in various [p. 518] parts of Europe, and they exist concealed by younger formations over considerable areas where they are not seen.Fig. 384 represents the general geographic relations of land and water in Europe during this period. The submerged area represents in a general way the area where the Ordovician formations are present.
The formations of the European Ordovician are largely fragmental, being made up of shales, sandstones, graywackes, etc., with which there is associated relatively little limestone. In this respect the Ordovician of Europe is in contrast with that of North America.
The system is represented in the British Isles by great thicknesses of strata (something like 24,000 feet maximum).[13] Locally (Wales), nearly half the system is composed of igneous rock, consisting of sheets of lava and beds of fragmental igneous rocks of various sorts. In the north of England, the successive beds of igneous rock, partly lava-flows and partly tuffs, not interstratified with aqueous sediments except near the base and summit, suggest that the eruptions took place, in part at least, on land. In Wales, [p. 519] on the other hand, the igneous rocks are interstratified with sedimentaries, and are therefore thought to have been ejected beneath water.[14] This is one of the most extensive, as well as one of the most ancient, volcanic tracts of Europe. From north England and Wales the system thins in all directions. In Scandinavia and Russia it has but a fraction of the thickness which it possesses in Britain. In southern Europe the system does not attain great thickness, and limestone is more abundant than in the north. The strata are exposed about various mountains where local disturbances have upturned them, and where erosion has cut off the beds which once overlay them.
In Bohemia, though the system does not appear at the surface [p. 520] over a great area, it has been made classic by Barrande,[15] who has studied its abundant fossils in great detail. The faunas of few areas in any part of the earth have been studied with equal care, or with richer results.
The Ordovician of Europe is generally conformable on the Cambrian, but over considerable areas it is unconformable beneath the Silurian. In the British Isles, the stratigraphic relations of these systems show that the Ordovician strata were elevated, folded, crumpled, and so metamorphosed as to greatly change their character at the close of the Ordovician period. In the highlands of northwestern Scotland, the dynamic action seems to have been exceptionably severe. The strata here were not only folded, but the folds were overturned, and a series of nearly horizontal faults or thrust planes developed. Locally the thrust was as much as ten miles,[16] and had for a result, the burial of the Ordovician strata, sometimes without metamorphism, by the Cambrian and even the Archean rocks. Over the greater part of the European continent, on the other hand, orogenic disturbances do not appear to have taken place at the close of the Ordovician. In Europe, as in America, the great disturbances took place where thick bodies of sediment had been accumulated (or else the beds were greatly thickened by the disturbances).
In other continents the Ordovician strata have not always been separated from the overlying Silurian, but they are known both in Australia and China.
Duration and Climate
The duration of the Ordovician is perhaps no better known than that of the Cambrian, but the period was probably somewhat shorter than its predecessor.
Neither in Europe nor in America is there decisive evidence that climatic zones were distinctly marked. All that is known of the life of this area would seem to indicate that the climate was much more uniform than now throughout the areas where the strata of the period are known. The fact that the Ordovician rocks have [p. 521] been identified in the far north (in North Devon, the west coast of King William’s Land, Boothia, etc.) by fossils akin to those of low latitudes, indicates that the climatic conditions of North America and Europe must have been less diversified than now. This apparent lack of diversity of temperature through wide ranges of latitude is one of the unexplained problems of geology. Its solution is possibly to be found in a much higher average temperature of the ocean, due to a deep circulation the reverse of that which exists now.[17] If the body of the ocean-water was relatively warm (instead of cold as now), it would have done much to counteract the effect of slight insolation during the cooler part of the year.[18]
Just as no great physical change took place in the passage from the Cambrian to the Ordovician period, so there was no pronounced break in the succession of life. The time from the beginning of the Cambrian to the close of the Ordovician appears to have been one long eon of progressive development and expansion of life, and its division into two nominal periods is artificial rather than natural.
The fossil record of the Ordovician is fuller than that of the Cambrian. This is due partly to an increase in fossilizable forms, partly to an increase in numbers of individuals, and partly to better conditions of preservation.
The general aspect of life was cosmopolitan, though it was not the same everywhere. It varied with the physical evolution of the continent, and largely as the result of it. The variations assumed three general phases: (1) adaptation to the immediate physical environment, particularly the nature and depth of the sea-bottom (edaphic adaptation); (2) modification by auto-evolution within restricted areas isolated by barriers (provincial evolution), and (3) modification toward a universal type through intermigration (cosmopolitan development) .
[p. 522]
(1) Edaphic modification. Rocky, sandy, muddy, and calcareous bottoms had their appropriate life, as did also tracts of shallow and deep water, and areas dominated by other special conditions. The assemblages adapted to these special conditions were not altogether unlike, for not a few forms, particularly freeswimming species, were indifferent to these conditions.
(2) Provincial modifications. Although the sea covered a large part of the continent, affording facilities for the migration and mingling of faunas, there was still evidence of some separation into zoological provinces. This was probably due partly (1) to barriers interposed by gentle warpings of the sea-bottom producing emergent tracts and tracts of excessive depth, and partly (2) to barriers constructed by the sea itself, in the form of shoals, bars, and spits. Provinces may have been defined also by (3) ocean-currents with their attendant differences in temperature, and they may have been due (4) to variations in the salinity of the waters. Provinces due to surface warpings seem to have been most marked in the Appalachian tract.[19]
(3) Cosmopolitan development. Notwithstanding the local and provincial modifications just noted, the progress of the Ordovician life on the American continent seems to have been, on the whole, in the direction of cosmopolitanism. This was due, primarily, to the wide development of the epicontinental seas, which gave a broad field for the evolution of marine life, and permitted free migration. A cosmopolitan tendency is particularly marked in the great interior of the continent. These statements apply chiefly to the shallow-water faunas. The deep-sea beds of the period are inaccessible.
The Ordovician system contains an exceptionally large number of fossils of free-floating graptolites (Fig. 394).[20] Their remains arc mingled with the fossils of the shallow-water life, showing that pelagic life swam freely over the epicontinental seas. The Ordovician graptolites are nearly identical in Europe, North America, [p. 523] and Australia, so that the range of the graptolite species was oceanwide. The history of individual species was not long, geologically speaking, and hence the succession of species is well suited for marking the progress of events in all parts of the ocean. During the lifetime of the graptolites (limited to the late Cambrian, Ordovician and Silurian), a score of successive zones, each characterized by particular species, have been identified. One of these zones falls in the Cambrian, eight in the Ordovician, and eleven in the Silurian. If these be taken as chronological bench-marks, the successive horizons of the different continents may be correlated accurately, and the progress of life in the various quarters of the globe referred to a common standard.
The Record of Marine Life
The known fauna of the Ordovician was made up almost wholly of marine invertebrates, among which trilobites and brachiopods [p. 524] held the leading places. The brachiopods were most numerous, the trilobites highest in organization, and the cephalopods most powerful. But the foreshaclowings of a new dynasty were at hand, for the remains of fish have been found in the strata of this system.
Trilobites and other crustaceans. The rise and fall of the trilobites is shown in the curve of Fig. 385. Their climax in the Ordovician appears to have been reached by a rapid ascent, which was followed by a more gradual decline. More than half of all known genera of trilobites are represented in the Ordovician system, but only a few lived over from the Cambrian. In the next period the [p. 525] numbers fell off a full half, and this decline continued until the tribe became extinct. The general aspect of the trilobites at the high tide of their career is fairly illustrated in Fig. 386. Their eyes were, as a rule, more prominent and better developed than those of the Cambrian species. There was little or no increase in average size. Some individuals reached a length of 18 inches and ranked among the giants of the group, but this size was equaled and even surpassed by some of their Cambrian forebears.
Besides the trilobites, the crustaceans were represented by a few inferior forms, such as ostracodes (Fig. 372) and cerripeds.
The Cephalopods. The largest, most powerful, and perhaps most predaceous of the known forms of Ordovician life were the cephalopods, which seem to have developed into prominence with extraordinary suddenness. Unless the fishes, of which little is [p. 526] known, contested their supremacy, they were doubtless the undisputed masters of the sea. Their relics first appear at the time of the transition from the Cambrian to the Ordovician, but they were then so far advanced and so widely differentiated from allied forms as to render it probable that they had already lived a long time. Their general aspect is seen in Fig. 387. The dominant form, as well as the most primitive one, was the Orthoceras (Fig. 387, c and f), whose shell consisted of a long, straight, gently tapering cone divided into chambers by plane septa, and connected by a central tube (the siphuncle). Even in the Ordovician period there was a a wide departure from the ideal simplicity of this genus. There were curved forms and coiled forms, some of which resemble the Nautilus of to-day (Fig. 387, e). Straight forms predominated, however, and the sutures (junctions of the septa with the outer shell) were simple. In later periods the sutures vary widely, and marked, in a very tangible way, the progress of the class. The size attained by the Ordovician cephalopods was probably never surpassed by representatives of the class. Some of the shells were 12 or 15 feet in length, and a foot (maximum) in diameter. From this great size they ranged down to or below the size of a pipe-stem.
[p. 527]
Other Molluscs. The gastropods were well represented in the early Ordovician fauna by diverse forms (Fig. 388). Few types of early Paleozoic life so closely resembled their modern relations. The pelecypods were subordinate to the gastropods both in numbers and range. Representative forms are shown in Fig. 389. Like their modern relations, the Ordovician pelecypods seem to have been fond of muddy and sandy bottoms, for they are rather rare in the limestone beds of the early and middle Ordovician. They increase in abundance as the deposits grade into the shales of the later Ordovician.
[p. 528]
The Brachipods. The lower, inarticulate forms of brachiopods which predominated in the Cambrian, continued through the Ordovician (and to the present time), but the higher, articulate forms greatly outnumbered them. The expansion of the articulate types was attended by a progressive evolution of the mode of articulation.
[p. 529]
In some the length of the hinge was increased, apparently affording a better means of resisting the attempts of their enemies to reach them by sliding or rotating the valves past one another (i and p,Fig. 390), while in others the margins of the valves were notched so that the valves interlocked n. The latter device was usually best developed in the shells of narrow and weak hinge-line, where it was most needed. In addition to these devices for preventing the opening of the shell, there was generally a thickening of the shells, and in many cases a ribbing of the exterior, giving strength without needless weight. All these devices seem to imply that the enemies of the brachiopods had increased in effectiveness, but the abundance of the brachiopods implies that their enemies did not gain the mastery. A comparison of the figures of Ordovician and Cambrian brachiopods (Figs. 390 and 373) will illustrate, in some degree, their changes.
[p. 530]
The Bryozoans. The bryozoans (Fig. 391), kin to the brachiopods (p. 945) were very unlike them in external form, in habits, and in their hard secretions. The bryozoans lived in colonies, connected by a common mantle which secreted calcareous material to form the framework of the colony. These secretions so closely resemble coral that they have often been mistaken for it. The bryozoans became abundant in the middle and later portions of the period, when their secretions contributed much to the limestone.
[p. 531]
[p. 532]
The Echinoderms. One division of the echinoderms, the cystoids, reached its climax before the close of the Ordovician period; another, the crinoids, became prominent, and others (asteroids, ophiurians, and echinoids, p. 945) had made their appearance. The cystoids, (a, b, and c,Fig. 392) with their irregular forms, were the most primitive, and gave place in time to the more symmetrical crinoids (Fig. 392, d to k), which may be likened to star-fishes turned face uppermost and fixed to the sea-bottom by a calcareous stem attached to the center of the back. The crinoids so closely resembled a flower in form, that the familiar name “sea-lily” is not inappropriate. The crinoids were excellent subjects for fossilization, save that, after the tissues decayed, the constituent hard portions fell apart easily so that perfect specimens are rare. Some limestone is made up largely of crinoidal fragments.
The structure of the cystoids (Fig. 392, a to c) was similar to that of the crinoids, but the body was unsymmetrical both in form and in the arrangement of the plates. Little can be said of their evolution, for their forms are so heterogeneous and their functions so little known that it is not clear what constituted progress. The other echinoderms attained their principal development later.
The coelenterates. Corals are few in the lower part of the system, and though more abundant in higher beds, are nowhere a leading element in the fauna. Most of them belonged to the simpler horn-shaped type (Fig. 393, a), but compound and colonial corals were present. The most important development of the coelenterates was the rise of the graptolites (Fig. 394), whose important function in correlation has been referred to.
Other forms. Sponges were present and sometimes attained notable size (Fig. 395) . The record of annelids is more meager than in the Cambrian, perhaps because the calcareous sea-bottom of the Ordovician was less congenial to them than the Cambrian sands. They are represented by burrows and by teeth (Fig. 396). Protozoans were probably present, but their minute and fragile shells can be recognized only with some uncertainty.
[p. 533]
Fishes. Fragmentary fossils of fishes constitute the most striking innovation in the record of the marine life of the Ordovician periocu These have been found in a few localities only, notably near Canyon City, Colo., and in the Bighorn Mountains of Wyoming.
Implied life. If we inquire what forms other than those fossilized are necessary to round out a rational assemblage of life, a briefer answer may be given than in the case of the Cambrian life, for the Ordovician fauna was a nearer approach to a theoretically complete assemblage. As in the Cambrian, a vast supply of unrecorded vegetation must be postulated as a food-supply. To provide for organisms that preyed upon one another in succession, from plants up to the master forms of the predaceous animals, there were doubtless many species not now known. The defensive investitures of the lower forms, not fully accounted for by the known Cambrian species, are now much more nearly explained by the prevalence of cephalopods and the presence of fishes. The armors of these dominant forms may have been defensive against their own kind. The fact that vegetal and animal tissues are not represented among fossils, save in exceptional cases, probably signified that the bacteria concerned in the decomposition of organic matter were abundant.
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Ecological, social, and mental development. It seems clear that the adaptation of the various forms of life to one another and to their physical environment had reached a higher stage of adjustment than in the Cambrian, an adjustment not greatly inferior to that which now prevails among the corresponding orders. It is not improbable that the mental development also approached somewhat nearly that now possessed by corresponding types. Higher types within the same orders have been developed since in many cases, and probably higher mental functions; but some of the Ordovician forms have since suffered degeneration. The Ordovician ancestors of the barnacle, for example, a free-moving, active form, was doubtless superior to his sessile descendant of ill-repute. The sum total of ecological adaptation and of social and mental development, on the average, seems to have advanced with each era.
The Record of Land Life
Plants. There are strong theoretical reasons for believing that land plants abounded, but only a few relics doubtfully interpreted as land plants have been found, and they reveal but little.
Insects. The oldest relic of insect life now known is a rather obscure wing found in the graptolite shales of the Upper Ordovician of Sweden. It is referred to the order of Hemiptera (bugs). Not enough is preserved to show fully the nature of the insect, but the existence of any flying insect of this sort implies the presence of vegetation, and of atmospheric conditions suited to active, airbreathing organisms.
Succession of faunas. There was a succession of Ordovician faunas, somewhat unlike one another, just as there was a succession [p. 535] of Cambrian faunas. These may be distinguished roughly as the Lower, the Middle, and the Upper Ordovician faunas. In some places, the late Cambrian faunas and the early Ordovician faunas merge into one another without sharp definition. In general, the Mid-Ordovician fauna was more prolific than that which preceded, if we may judge from the fossils. The Mid-Ordovician fauna, too, was distinctly cosmopolitan. The Upper Ordovician fauna was similar to its predecessor, from which it descended, but the prevailing muddiness of the bottom of the late Ordovician seas seems to have had some influence on the life, and clear-water forms were less dominant.
The successive sub-faunas of the period were much the same in other continents as in America. Most genera were the same, but the species were, as a rule, different, though they often bore a close resemblance to the American species. In northwestern Europe, with which the means of migratory communication seems to have been freest, not a few common American species flourished. In Asia, so far as present limited information goes, the species were nearly all different, the wide-ranging graptolites excepted. The stages of progress in the shallow-water faunas of the Old and New World, are to be regarded as parallel rather than identical. The evolution in Europe, where alone details have been well worked out, was usually on narrower lines than that of the American interior, for the obvious reason that the epicontinental seas were more limited and more interrupted by barriers.
Map Work. The folios which serve for the study of the Cambrian system (p. 506) are serviceable also for the Ordovician. In the folios, however, the Ordovician and Silurian systems are put together on the maps, under the name Silurian; but the texts of the later folios distinguish between the two.
There are local unconformities between these systems, as in some parts of New York, and they may be more wide-spread than has been supposed. ↩︎
Question has recently been raised as to the propriety of including the Richmond beds in the Ordovician. It has been suggested that they are perhaps the equivalent of the Medina, and if so they belong with the succeeding system. Hartnagle, N. Y. State Mus. Bull. 107, 1907. In Illinois, beds of Richmond age are unconformable on the underlying Ordovician. Weller, Jour, of Geol., Vol. XV, p. 519; and Savage, Am. Jour. Geol., Vol. 125, p. 431, 1908. ↩︎
It is now held by some that a portion at least of the Hudson River shale of the Mississippi basin (Maquoketa of Iowa, Illinois, etc.) is the equivalent of the Richmond beds farther east. Its classification with the Ordovician is therefore subject to question. (See footnote, p. 508). ↩︎
For local details in the Appalachians, see the folios of the U. 8. Qeol. Surv. On the maps of the folios, the Ordovician is classed with the Silurian under the latter name. The text of the folios frequently distinguishes between the Lower Silurian (Ordovician) and the Upper Silurian (Silurian). ↩︎
The subdivisions mentioned here are those of the Maynardsvillc, 'IYnn., folio, U. S. Geol. Surv. ↩︎
See, for example, the New York City, Holyoke (Mass. -Conn.), and [awley (Mass.) folios, U. S. Geol. Surv. Compare with folios of (1) the Appalachian Mountains, (2) the interior, and (3) the western part of the United States. ↩︎
Hayes and Ulrich, Columbia (Tenn.) folio, U. S. Geol. Surv.; also Foerste, Geol. Soc. of Am., Vol. XI, p. 604, and Vol. XIII, p. 631; Science, New Ser., Vol. X, p. 488, and 24th Ann. Rept., Dept. of Geol. and Nat. Hist., Resources of Indiana, 1899. ↩︎
Branner, Am. Jour. Sci., Vol. IV, 1897, p. 357. This very suggestive article has bearings on many questions besides the Ouachita Uplift. ↩︎
Orton, 8th Ann. Rept., U. S. Geol. Surv.; Phinney, 11th Ann. Rept.; also the reports of the State Geol. Surv. of Ohio and Indiana. ↩︎
Chamberlin, Geol. of Wis., Vol. IV, 1879, pp. 365-568; Calvin and Bain, Iowa Geol. Surv., Vol. VI, and Grant, Bull. XIV, Wis. Geol. Surv., 1906. ↩︎
Winslow, Missouri Geol. Surv., Vols. VI and VII. ↩︎
Hayes, Columbia (Tenn.) folio, U. S. Geol. Surv. ↩︎
This measurement is doubtless subject to the strictures sel forth on p. 461. ↩︎
Geikie, op. cit., pp. 946 and 949. ↩︎
Système Siluriene de la Bohème. ↩︎
Quart. Jour. Geol. Soc, 1884 and 1888. ↩︎
Chamberlin, Proc. Am. Phil. Soc, Vol. XLV, 1906, pp. 1-11. ↩︎
Chamberlin and Salisbury, Earth History, Vol. Ill, pp. 437-445 ↩︎
Paleozoic Seas and Barriers; E. O. Ulrich and Charles Sehuchert; Rept. N. Y. State Paleontologist, 1901, pp. 633-668. ↩︎
It is not universally agreed that all graptolites were floating forms al nil stages, but there seems to be little doubt that they usually were in their young stages at least. ↩︎