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The physical changes which brought the Ordovician period to a close marked also the inauguration of the Silurian. These changes included (1) movements which affected small areas intensely, and (2) movements which affected broad areas slightly. From the standpoint of continental history, the latter were the more important. These changes were doubtless accomplished slowly, and after they had taken place, the area of land in North America was greater than at any time since the early Cambrian. The increase in land meant lengthened streams, and presumably increased erosion.
Could the distribution of land and water at the beginning of the Silurian be denned accurately, it would define also the areas where the earliest marine sedimentation of the period took place, and, in a general way, the areas where sedimentation was rapid and where slow, for then, as always, the rate of sedimentation must have stood in more or less definite relation to the shore-lines. It is safe to assume that at the opening of the Silurian period beds of clastic sediments were accumulating about the immediate borders of the lands, and as far out as waves and currents were able to transport abundant detritus, and that elsewhere sediments of organic origin were relatively more important. Though sedimentation was interrupted in the regions which emerged from the sea during the transition from the Ordovician period to the Silurian, such interruption was not universal, and the Silurian strata are locally conformable on the Ordovician in the continent, and generally, it is to be presumed, in the ocean basins.
[p. 537]
Like the preceding system, the Silurian may be divided into three principal series, as follows:[1]
Each of these series is subdivided into several formations, but the subdivisions of one place do not fit another. A brief sketch of the nature and distribution of these principal subdivisions of the system affords an outline of the history of the continent during the period.
Silurian of the East
The Oswegan series. The principal subdivisions of this series in the east are the Oneida and the Medina. Both appear at the surface south of Lake Ontario, and perhaps in the western part of the Appalachians farther south.[2] The Oneida consists of conglomerates and sandstone, and the Medina of sandstone and shale. The sediments of these formations appear to have been deposited in a shallow interior sea, as shown by their fossils, and by the crossbedding, the ripple-marks, etc., which affect its layers. The Medina formation extends farther west than the Oneida, reaching eastern Ohio and Ontario. Its distribution, as compared with that of the Oneida, points to a subsidence of the eastern interior during the early part of the Silurian period. Both formations are probably continuous beneath younger strata over considerable areas south of Lake Ontario and the MohawTk valley, and west of the Appalachians.[3] Nothing is knowm of the sediments which were accumulating [p. 538] along the eastern border of Appalachia at this time, nor of those about the other land areas.
The Niagaran series. The Clinton formation overlies the Medina conformably, but has a wider distribution. Westward, it extends to Lake Huron and Indiana, and perhaps to the Mississippi. If this is the case, it is represented by beds which have been classed with the Niagara limestone. The formation occurs in the Appalachians as far south as Alabama and Georgia. The fossils of the formation in the Appalachians are so unlike those of the interior, as to lead to the inference that the sites of sedimentation in the two areas were not freely connected. Beds of Clinton age have been recognized in Nova Scotia and at a few other places northeast of the United States. Marine sedimentation was probably continuous here through the Ordovician and Silurian periods.
The variations in the character of the Clinton beds in different localities are significant. In the Appalachian Mountains, the formation is largely of sandstone and shale. In western New York and farther west, much of it is limestone. The limestone does not mean that the water was necessarily deep, but rather that it was so nearly free from clastic sediments that the shells, etc., constituted the principal part of the deposit made. Shell-bearing life may be just as abundant where sand and mud are accumulating as elsewhere, but in this case the product is not limestone, but sandstone, shale, etc., containing shells. Bryozoan reefs, resembling coral reefs, occur in the formation in western New York.[4]
One of the notable features of the formation is its content of iron ore, generally in the form of hematite (Fe2O3) . The ore is often made up of small concretions which so resemble flaxseed in size and shape as to have suggested the name “flaxseed” ore. Locally it is known also as “fossil” ore from the abundance of fossils which it contains. The ore is known at many points between New York and Alabama, as far west as Wisconsin, and as far northeast as Nova Scotia. The ore is interst ratified with other beds of the formation, and is usually believed to have been accumulate. I by chemical precipitation in lagoons or marshy flats.
Like the Oneida and Medina formations which preceded, the [p. 539] Clinton beds have not been identified in the western half of the continent.
The Clinton formation was succeeded by the Niagara formation (subdivided in New York into Rochester shale, Lockport limestone, and Guelph dolomite, p. 537), which extends farther west than any of the preceding Silurian formations, showing that the progressive submergence of the earlier epochs still continued in the upper Mississippi basin. The falls of Niagara River are over the limestone of this series (Fig. 113). North of Missouri, the formation is not known to occur far west of the Mississippi, but it extends into Missouri, Arkansas, and perhaps even to the Arbuckle Mountains of Oklahoma. It is also found in the trans- Pecos region of Texas. It was in this epoch that the submergence of the upper Mississippi basin reached its maximum, so far as this period is concerned. The southern border of the interior sea is not known, but it appears to have been separated from the ocean in this direction, by a land barrier somewhere in the Gulf-State region (Fig. 397). The barrier was probably south of Tennessee, as Niagaran limestone occurs in the western part of that state.
A significant feature of the distribution of the Niagara formation is its great development in high latitudes. It occurs in patches in Manitoba, west of Hudson Bay, and at numerous points farther north, up to latitude 80°. The patches appear to be remnants of a once continuous formation, and since the fossils are much the same throughout, and very like those of northern Europe, it is inferred that there was water connection between the Mississippi basin id northern Europe by way of the Arctic islands, which permitted the intermigration of the shallow-water sea-life of the two regions.
East of the Appalachians and west of the Mississippi the distrimtion of Niagaran strata is not well known. They probably >ccur in New Hampshire and Maine, and in the Provinces between these states and the St. Lawrence. In Nova Scotia, the Niagaran jries is represented by shale, affording another illustration of the fact that different sorts of rock may be accumulating in different igions at the same time. The exact equivalent of the Niagara formation has not been identified with certainty in the West.
West of New York the formation is mainly limestone. Perhaps no preceding formation of limestone except the Trenton is so widespread. It is the oldest formation in which well-developed coral reefs .have been identified, though coral-secreting polyps had lived before (p. 532) . The reefs are known in eastern Wisconsin, Indiana, and elsewhere.
[p. 540]
[p. 541]
These reefs and the deposits adjacent to them illustrate as clearly as anything among the ancient formations, the origin of several varieties of limestone. The reefs themselves are composed of the commingled relics of the life that grew upon them. Great masses of coral sometimes stand erect in the rock just as they grew, having escaped destruction during their burial in the growing reef. In other instances, the coral masses in the limestone are fragmentary, broken and worn by the waves. With the larger pieces of coral there is coarse and fine detritus, the product of coral comminution. These combine to make up the mass of the reef-rock. Between and about the reefs a mixture of shell and coral fragments and calcareous sands accumulated, growing finer as distance from the parent reef increased and the slope of the bottom became more gentle, these materials grading finally into calcareous mud, which was spread widely over the sea-bottom about the reefs. This fine calcareous sediment ultimately gave rise to compact white limestone.
Thickness and structure. Unlike the preceding formations of the Silurian, the Niagara is not thicker in the east than elsewhere. In the east, indeed, where the formation is exposed, it has a thickness of but 100 to 300 feet, while in Wisconsin it attains a maximum of 800 feet (perhaps including some Clinton), all of which is limestone. While the Niagaran beds of the interior are in general nearly horizontal, they are frequently domed so as to give the beds a high angle of dip (Fig. 398) . This is true, for example, at various points about the southern end of Lake Michigan.
[p. 542]
The Cayugan (Salina) series. The Salina formation, which overlies the Niagaran in parts of New York, Pennsylvania, Ohio, Michigan, and Ontario, is much less widespread, and its limited distribution points to the emergence of a considerable area in the Mississippi basin at the close of the Niagaran epoch.
The Salina series embraces several varieties of rock, including conglomerate, sandstone, limestone, shale, and rock salt. With these formations, some gypsum, the usual accompaniment of salt beds, is associated. Shale is the most abundant, and seems to have originated after the fashion of shales in general, but the fewness of its fossils seems to point to deposition under conditions unfavorable for life.
The salt is widely distributed. It occurs at many points in New York within an area 9,000 to 10,000 square miles in extent. Single beds of it are locally 40 to 80 feet thick. Several beds sometimes occur one above another, interstratified with other sorts of rock, and their aggregate thickness sometimes reaches as much as 100 feet. Near Cleveland, four salt beds, 50 feet and less in thickness are interstratified with 500 feet of shales. In some places, therefore, the salt makes a very considerable fraction of the total thickness of the series.
The salt beds seem to imply the existence of great lagoons or inclosed seas, in which the Salina series was deposited. Had the climate of this region been as moist as now, these lagoons could not have been abnormally saline. Occasional incursions of the sea, bringing in new supplies of salt water, followed by periods when the lagoons were cut off from the sea, and when they suffered rapid evaporation, would seem to meet the conditions demanded for the formation of the salt. So also would a slight continuous connection with the sea, such that the inflow of sea-water into the basin did not balance the excess of evaporation over precipitation in the basin area. A bed of salt 40 feet thick implies the evaporation of sonic 3,000 feet of normal sea-water. Much of the salt of commerce which comes from New York is not derived immediately from the salt beds, but from the waters of salt wells.
The limestone of the Salina proper is largely contemporancoi with the shales and salt beds. It is thickest where thoy are thin, [p. 543] and thin where they are thickest. It contains few fossils, and it is thought that it may be a chemical precipitate. Its relations to the shales and salt beds are such as to indicate that the areas of accumulation of the several sorts of rock material were shifted from time to time, as if by gentle changes of level of land or water.
Above the Salina proper of New York, there is a thin (150 feet maximum) series of limestones, the Waterlime (Cobleskill, Rondout, Manlius, etc.), generally regarded as a part of the Silurian system. The name Waterlime has reference to the fact that the limestone is the source of hydraulic cement, though it is by no means everywhere useful for this purpose, and many other limestone formations are similarly used. The Waterlime is more widespread than the Salina, extending westward through Ohio to Indiana and Wisconsin. Both its distribution and its character show that the eastern interior was more generally submerged than during the deposition of the salt-bearing series which preceded.
In the northern Appalachians there is a conglomerate formation (Shawangunk, N. Y., Green Pond Mountain, N. J.), formerly classed as Oneida, which is now regarded as contemporaneous with the Cayugan series. Until recently, this conglomerate was not known to contain fossils, but their discovery has shifted the classification of the formation from the early Oswegan to the later Cayugan series.[5] The materials of this conglomerate are mostly quartzose, and seem to have been derived from lands to the east. They have been thoroughly indurated by cementation, so that the formation is exceedingly obdurate. The outcropping edges of its tilted beds constitute the crest of the Kittatinny range in New Jersey, and its continuations in New York and Pennsylvania. Silurian rocks have been recognized recently among the gneisses of Connecticut.
The Helderberg formation, formerly regarded as a part of the Silurian system, is here classed with the Devonian.
Silurian of the West
At various points in the West, there is a series of sedimentary beds, poor in fossils, between the known Ordovician below and the Devonian above. The character of the fossils being indecisive, [p. 544] the age of the beds is open to question. Soje of them may be Silurian. If the Silurian is really absent from all the areas where its presence is not now known, it would appear that a large part of western North America was land during the Silurian period. Silurian beds are however known in Southern California, Nevada, Utah, and Alaska, and perhaps in the Canadian Rockies, and its distribution may be more widespread than has been supposed.
Summary
As in the case of all preceding systems of the Paleozoic, the greatest thicknesses of Silurian strata (estimated at about 5,000 feet, maximum) occur in the Appalachian mountain region. Over the interior, the system is relatively thin, being measured by hundreds of feet rather than thousands. In keeping with these variations, the system is largely of clastic sediments of shallow-water origin in the Appalachian belt, while in the interior it is largely of limestone. The site of sedimentation in the east was a sort of trough (the Appalachian trough) shut off from free communication with the interior sea, but connected with the Atlantic, perhaps by way of the present Chesapeake region. Since most of the sediments of this trough were deposited in shallow-water, they are usually thought to indicate that the trough was sinking at a rate comparable to that at which the sediments accumulated. This view may, however, need to be qualified as suggested on page 461. So far as sinking took place, the thick sediments may have been its cause, or one of its causes. With the down-warping of the trough, there may have been up-warping of the adjacent land area which supplied the sediments.
The history of the Silurian period, as now understood, involves, (1) a general submergence of the eastern part of the United States west of Appalachia, by which the sea became more and more widespread until the close of the Niagaran epoch; (2) a partial withdrawal of the sea from the same area during the Salina epoch; and (3) an extension of the sea at the close of that epoch. There were doubtless many minor oscillations of level which have not been determined.
Former extent and general stratigraphy. The present margins of the several Silurian formations are not their original margins, [p. 545] for their exposed parts have suffered erosion, and the erosion of dipping beds shifts their outcrops. In some localities there are data for estimating something of the former extension of the formations. In Wisconsin, for example, remnants (outliers, a, b, c,Fig. 399) of the Niagara are found far beyond the main body of the formation as it now exists. These outliers fix at least a minimum limit to the original extension of the formation.
Igneous rocks. At few points in North America have igneous rocks of Silurian age been identified. The Silurian formations are sometimes affected by igneous intrusions, but the date of the intrusions is generally uncertain. Some of the igneous rocks of New Brunswick are thought to be of Silurian age, and perhaps some of those of Nova Scotia and Maine.[6]
Close of the period. The geographic changes at the close of the Silurian were less than those at the close of the Ordovician, and the Silurian system is perhaps less distinctly separated from the Devonian above than from the Ordovician below.
Climate and duration. There is nothing to indicate great diversity of temperature in the Silurian period, and much to suggest [p. 546] that uniformity extended through great ranges of latitude, for the fossils of warm-temperate regions are in part the same as those in Arctic regions. Some regions appear to have been temporarily very arid. The Silurian period was perhaps comparable in duration to the Ordovician.
Foreign Silurian
In Europe the Silurian strata have a distribution similar to that of the Ordovician, though they are wanting in some regions where the latter are present. The fact that the Silurian strata do not appear at the surface over wide areas does not indicate their general absence, so much as their wide-spread concealment. In most of the northern part of Europe, outside of Britain, the system has been little deformed. In the southern part of the continent, the Silurian formations appear in small areas only amidst formations of lesser age. In contrast with the Silurian rocks of the northern province, those of the southern are much deformed.
The Silurian formations of Europe, especially of the northern province, are more largely composed of limestone than those of the Ordovician, suggesting clearer seas.
Geographic changes took place in Europe at the close of the period as shown by the unconformity between the Silurian and Devonian systems in some places (Great Britain and Ireland), though conformity is the rule.
The Ordovician and Silurian of other continents have not been generally distinguished. The equivalents of the two systems as distinguished in Europe and North America probably occur in all the less well-known continents.
The extensive withdrawal of the sea from the surface of North America at the close of the Ordovician -period reduced the area of shallow-sea water available for the life which iHHMlod it . The severe repressive evolution which followed was the great biological feature of the transition from the Ordovician to the Silurian. With the re-invasion of the interior by the mid-Silurian sea, there followed an expansional evolution of the shallow-water fauna, which constitutes the great biological feature of the middle of the period. Toward the close of the period there was another restriction of the [p. 547] epicontinental sea, complicated with intense salinity in the eastern interior region, and there followed a second repressive evolution by which the fauna passed into the Devonian type.
Theoretically, the history of the land life should have been the reciprocal of that of the sea; for as the sea contracted, the land expanded, and an expansional evolution of land life should have run hand in hand with the restrictional evolution of the sea life. This was probably the fact, but the record of the land life is too meager to , demonstrate it. In so far as the climate was arid, it was unfavorable for abundant land life.
The Transition from the Ordovician
Of the shallow-water life of the early Silurian there is but meager record. The eastern shore of the continent was then far out on the borders of the continental platform, and the deposits there are buried and inaccessible. The western border may have been submerged, but the fauna there is little known.
Aside from the lessened area favorable for shallow-water life, the conditions were probably less favorable, area for area, than before, for the wash from the land was presumably increased. The increased detritus brought to the sea probably inhibited some forms of life, injured others, and helped but a few. Some of the basins and bays were doubtless too fresh and some too salt, and some may have varied unfavorably in salinity. These general considerations may explain the meagerness of the faunas of the early Silurian strata. But conditions were not adverse everywhere. In the Gulf of St. Lawrence, Ordovieian species lived on for varying lengths of time, and mingled with Silurian species as they developed, and so recorded the transition. This appears to have been the breeding-ground of one of the provincial phases of the Silurian fauna, but it is not probable that it was the only one. The main Silurian fauna of the interior apparently did not spring from that of the Atlantic border, but developed somewhere at the north, or migrated from Europe by a northerly route.
The Expansional Stage and the Mid-Silurian Fauna
As the sea slowly overspread the continent toward the middle of the period, increasing room and more congenial conditions for [p. 548] most forms of shallow-water life resulted in an expansional evolution which produced the Niagara fauna. The families and classes were much the same as in the Ordovician period, but most of the genera were new, and nearly all the species. In general there was a biological advance, though this was not true of all classes. Only the more conspicuous, features of the changes will be noted.
The echinoderms. A distinguishing feature of the Silurian fauna was the rich and varied development of the echinoderms, involving at once the rise or the decline of previous forms, and the introduction of new ones. The great feature of the period, in connection with the echinoderms, was the rise of the crinoids. They attained such abundance in certain congenial localities that their fragments formed the larger part of the limestone. These spots were veritable “flower-beds ” of “ stone lilies,” where beautiful and varied forms grew in groves, as it were. The assemblage of species at each of these localities had its own peculiarities, but the genera were the same or similar. A few Silurian crinoids are shown in Fig. 400, but no limited number of figures can do justice to their variety and beauty. Notwithstanding various signs of progress, many of the more primitive characters remained, indicating that the class had not yet reached its climax.
Cystoids were still abundant, and the true blastoids now appear for the first time. Starfishes appear to have made little progress and to have had no large place in the fauna, and the serpent-stars and the echinoids had even less. The slow development of these types which were prominent much later, possibly represents a general fact, viz., that great classes were really slow in their evolution, however suddenly they may seem to have come into existence. Perhaps the ascent of the cystoids and crinoids to their climaxes would be found to be as slow as those of their kin, if we could trace their history back to its beginning. A little greater imperfection in the fossil record would have eliminated all trace of the serpentstars and sea-urchins in these early periods, and would have made their appearance in abundance at a later period seem sudden and remarkable.
The brachiopods. The brachiopods stood the vicissitudes of the passage from the Ordovician to the Silurian with no loss of prestige, [p. 549] though they suffered an almost entire change of species and a verylarge change of genera. When it is considered that the brachiopods were among the most resistant and conservative of the invertebrates, this large change of species and genera emphasizes the stress of the conditions that controlled the transition, and its biologic importance. The Silurian brachiopods had gained in differentiation, and had made some notable advances in structure. On the whole they were more robust and gave more obvious signs of abounding vitality than before; but along with the progressive developments there were some retrograde modifications.
[p. 550]
The bryozoans. The coral-like bryozoans contributed much less to the Silurian limestones than to those of the preceding period. This was partly because the class had declined, and partly because the more massive types were replaced largely by more delicate and fragile forms.
The mollusks. The cephalopods appear to have remained tho most powerful inhabitants of the seas. The straight forms were still common, but curved and coiled ones were more numerous. Their shells were more highly ornamented than before, but they were still plain in comparison with some of their successors. The apertures of the shells of the Ordovician species were usually circular [p. 551] or oval, but in the Silurian species many of them were curiously constricted (b,Fig. 402), especially among the small curved and coiled species. The constriction appears to have been a protective device.
Although the gastropods were fairly well represented in the Cambrian period, and amply in the Ordovician, they did not increase greatly in the Silurian. They show advance in the preponderance of elevated spires, in increased variety of form, and some of them in greater size; but the older types were still plentiful.
The pelecypods (f, Fig. 402) were not so well represented in the mid-Silurian beds as in the Ordovician, perhaps because the calcareous bottoms were less congenial to them.
Corals. The prominence gained by the corals in suitable situations is one of the notable features of the Silurian fauna. In the Ordovician period, the simple forms predominated over the compound. The ratio was now reversed. Among the notable types was the unique chain coral (Halysites, Fig. 403, c), which had appeared in the Ordovician; the honeycomb coral (Favosites, a); [p. 552] the organ-pipe coral (Syringopora, b) ; and the cup coral (Zaphrentis, e). A most peculiar coral of the simple class (Goniophyllum, d) was quadrangular, and its top provided with a cover (operculum) of four triangular plates hinged to the four sides of the cup’s margin.
When closed they formed a pyramid over the cup (d,Fig. 403, only two of the opercular plates shown). This was a protective device unknown among modern corals.
With their increase in abundance, the corals acquired the habit of associating themselves together. This resulted in the formation of reefs. The known reefs appear to have been formed some [p. 553] distance from shore, and to have been of the barrier type. The reef-forming habit appears to have been local rather than general, for over large tracts corals are found scattered in a markedly distributive fashion.
The trilobites. No new families of trilobites appeared, though some new genera were added and many species; but these did not offset the disappearance of old ones, and the class, though still important, had already entered upon its decline numerically. The highest forms were, however, structurally equal, and perhaps superior, to any that preceded.
Other marine invertebrates. Sponges flourished. There was a prolific field of them in western Tennessee, where the conditions were not only congenial to their growth, but favorable for their preservation. The peculiar Receptaculites family (Fig. 405). whose affinities were long in doubt, was still present, though its climax was passed. The graptolites had lost the importance they had in Ordovician times, and by the end of the period neared extinction.
[p. 554]
Sea-worms are recognized through their jaws, tracks, and burrows, and by the calcareous tubes which some of them secreted.
The vertebrates. In the earlier and mid-Silurian deposits few relics of fishes have been found, and these few are very imperfect; but in the upper part of the system their remains are not rare.
Marine plants. Knowledge of marine plant life remains, as before, unsatisfactory. While theoretically it must have been abundant, only obscure markings of the fucoidal type and a few evidences of higher forms have been found, and their interpretation is more or less doubtful.
Foreign Faunas and Migrations
Deferring the consideration of the peculiar closing fauna for a moment, the relations of this rich and varied mid-Silurian assemblage may be noted. The general progress of life on other continents, so far as known, was similar to that on the American, but [p. 555] perhaps less pronounced and symmetrical. Where the forms on different continents were merely similar, it is at present an open question whether the similarity was due to intermigration, or to independent evolution along similar lines; but where the forms on separate continents are identical, especially if the species are peculiar and aberrant, it may be assumed that they had a common origin, and that migration is indicated. A striking case of this kind is presented by the quadrangular operculated coral already mentioned (Fig. 403, d), which is found with identical idiosyncrasies in the island of Gotland in the Baltic Sea, and in Iowa. The evidence of migration is strengthened by the presence in Gotland of three or more peculiar genera of crinoids that are found also in the upper Mississippi basin. Besides such special cases, many prominent and familiar species were common to America and Europe, and some of them are also found in Asia, Australia, and New Zealand. Migratory connections between North America and these lands may therefore be assumed. Since thirty or more species are known to be common to North America and Europe (the interior of North America and Sweden and Great Britain) , and since these embrace a wide range of genera of very different habits, there is a strong presumption of migration between North America and northwestern Europe.
It has already been stated that the interior sea of North America seems to have had no free communication with the sea to the east or south, and that its extension far to the west is doubtful. To the northward, on the other hand, the series of remnants of Silurian formations, probably once connected, point to a broad thoroughfare for shallow-water life between North America and Europe by way of Greenland.
The faunas that occupied the Appalachian trough, the St. Lawrence embayment, and the Atlantic coast appear to have had but limited connection with the fauna of the interior during the mid-Silurian epoch. In the present state of knowledge, the faunas of the Appalachian trough bear the aspect of provincialism.
The Closing Restrictional Stage
Following the luxuriant life of the mid-Silurian epoch, there came, in North America at least, a notable decline, due to the [p. 556] withdrawal of the epicontinental waters from the larger part of the interior, and to the conversion of the remainder into an excessively salt sea, in the deposits of which few fossils are found. The Waterlime beds represent a gradual return in the Salina basin of conditions hospitable to life. The fauna of these beds is limited, but radically unlike that of the Niagaran epoch. Most of the familiar marine types are absent from the later fauna, and its signal feature was an abundance of large crustaceans of types barely represented before. The most characteristic of these were the great Eurypterus (Fig 406, a) and the still more gigantic Pterygotus (b). The former reached a length of a foot and a half or more, and the latter attained a length of over six feet in the next period. These were gigantic dimensions for crustaceans, and were probably never surpassed. These giants among their kind seem clearly to have been aquatic, but whether they were inhabitants of salt or fresh water is not obvious. They are wholly extinct, and their habitat can only be inferred from their associations. In England, Sweden, and Russia, eurypterids are associated with marine fossils, but they are also associated with the seeds of land plants and with fish which, in the succeeding stage, seem to have occupied land waters chiefly. In the Devonian and Carboniferous periods, eurypterids are associated with land plants, scorpions, insects, fishes, and fresh-water amphibians, which seem to imply a freshwater habitat. In the light of these facts, the more common inference has been that they were originally marine forms, and became adapted later to brackish- and fresh-water conditions. An alternative inference is that they were originally denizens of the land waters, and that their remains were sometimes carried out to sea by streams, and thus fossilized with marine forms. Mollusks, crinoids, corals, and similar marine forms are almost entirely absent from the fauna of the Waterlime. The few brachiopods found are usually pauperitic, as though they lived in uncongenial conditions. The occasional presence of a few undoubted marine forms does not so much indicate that the waters were habitually saline, as that they were occasionally and partially so.
[p. 557]
It is at this time also that the earliest known scorpions appeared both in America and Europe. They were allied to the eurypterids. The European forms have been thought to be land species, though this has been questioned. The sting and the poison glands have been identified, and the significant name, Palceophonus, “ancient murderer,” applied in consequence (Fig. 406, c). The American species have been thought to be aquatic.
The presence of fishes emphasizes the peculiarities of this fauna. Except for their occurrence at a few points in the Rocky Mountains in the Ordovician, fish remains have not been found in America until this stage. In Europe a few fishes appear somewhat earlier, but nearly all the fish remains of the period yet found are in the very highest horizons of the Silurian, or in the deposits that form the transition to the Devonian, where they are associated with eurypterids and land plants, as well as marine invertebrates. It would appear that the fishes of the time were varied, and that they were the forerunners [p. 558] of the abundant fishes of the Old Red Sandstone of the Devonian. As the sand of this formation was probably deposited by land waters, the association of the fish with eurypterids and land plants carries some further presumption that the peculiar crustacean fauna lived normally in land waters. It is not to be overlooked, however, that in the transition beds the fishes are also associated with marine fossils, and there is no question that, before the close of the Devonian period, certain fishes at least were truly marine.
Map work. See note at end of Chapter XVII, page 535.
In this case, there is some infelicity in the use of the terms Lower Silurian, Middle Silurian, and Upper Silurian, for the subdivisions of the system, since Lower Silurian was long used as a synonym for Ordovician, and Upper Silurian for Silurian, as that term is here employed. ↩︎
Some of the formations formerly classed as Oneida in New York and New Jersey are now regarded as of Salina age (see p. 543). ↩︎
Perhaps the Richmond beds, the Maquoketa shales, etc. See foot-notes, pp. 509 and 510. ↩︎
Sarle, Am. Geol., Vol. XXVIII, p. 282. ↩︎
Hartnagle, Bull. 107, N. Y. State Mus. ↩︎
Williams, H. S… Jour, of Geol., Vol. II, pp. 16-18, and Penobscot and Rockland folios, U. S. Geol. Surv. ↩︎