pods to forty-seven Lamellibranchiata. In the antecedent Silurian rocks the relative numbers are still more in favor of the Brachiopods, whereas, in the more modern Carboniferous formation, the proportions are more than reversed, for there are of the Carboniferous Lamelli branchiata 282 species, and only 123 Brachiopoda. The reader will of course conclude from what was said at p. 414 that all these oolitic species were not living at one and the same time, there having been continual changes going on in the fauna from the period of the lowest to that of the uppermost member of the oolitic series; but the proportions of the two families of shells may be correctly deduced from the data above given. If we consult the same table to obtain the relative numbers of these same orders of mollusca in the oolites, we find 536 Lamellibranchiata and only sixty-nine Brachiopoda, these last therefore being reduced to nearly an eighth part of the whole bivalve fauna. If we then turn to the actual British seas, we observe that Forbes and Hanley give 220 living species of Lamellibranchiata and only five Brachiopods, the latter being reduced to a forty-fourth part of the whole fauna. As the lamellibranchiate mollusks have an organization of a more complex and higher grade, the fact of their increasing preponderance from the earliest to the latest times has been often cited, and not without reason, as favoring the theory of progressive development. Devonian Strata in the United States and Canada. In no country hitherto explored is there so complete a series of strata intervening between the Carboniferous and Silurian as in the United States. This intermediate or Devonian group was first studied in all its details, and with due attention to its fossil remains, by the Government Surveyors of New York. In its geographical extent, that State, taken singly, is about equal in size to Great Britain; and the geologist has the advantage of finding the Devonian rocks there in a nearly horizontal and undisturbed condition, so that the relative position of each formation can be ascertained with certainty. Subdivisions of the New York Devonian Strata, in the Reports of the Government Surveyors. These subdivisions are of very unequal value, whether we regard the thickness of the beds or the distinctness of their fossils; but they have each some mineral or organic character to distinguish them from the rest. Moreover, it has been found, on comparing the geology of other North American States with the New York standard, that some of the above-mentioned groups, such as Nos. 2 and 3, which are respectively 1500 and 1000 feet thick in New York, are very local, and thin out when followed into adjoining States; whereas others, such as Nos. 8 and 9, the total thickness of which is scarcely 50 feet in New York, can be traced over an area nearly as large as Europe. Respecting the upper limit of the above system, there has been very little difference of opinion, since the Red Sandstone No. 1 contains Holoptychius nobilissimus and other fish characteristic generically or specifically of the European Old Red. More doubt has been entertained in regard to the classification of Nos. 10, 11, and 12. M. de Verneuil proposed in 1847, after visiting the United States, to include the Oriskany sandstone in the Devonian; and Mr. D. Sharpe, after examining the fossils which I had collected in America in 1842, arrived independently at the same conclusion.* The resemblance of the Spirifers of this Oriskany sandstone to those of the Lower Devonian of the Eifel was the chief motive assigned by M. de Verneuil for his view; and the overlying Schoharie grit, No. 10, was classed as Devonian because it contained a species of Asterolepis. On the other hand, Prof. Hall adduces many fossils from Nos. 10 and 12 which resemble more nearly the Ludlow group of Murchison than any other European type; and he thinks, therefore, that those groups may be "Upper Silurian." Sir William Logan has shown that the fossils of the Gaspé limestones in Eastern Canada favor the same opinion, and demonstrate at least how difficult it is to draw a dividing line in that country between the Devonian and Silurian systems. Although the Oriskany sandstone is no more than 30 feet thick in New York, it is sometimes 300 feet thick in Pennsylvania and Virginia, where, together with other primary or paleozoic strata, it has been well studied by Professors W. B. and H. D. Rogers. The upper divisions (from the Catskill to the Genesee groups inclusive, Nos. 1 to 4) consist of arenaceous and shaly beds, and may have been of littoral origin. They vary greatly in thickness, and few of them can be traced into the "far West;" whereas the calcareous groups, Nos. 8 and 9, although in New York they have seldom a united thickness of more than 50 feet, are observed to constitute an almost continuous coral-reef over an area of not less than 500,000 square miles, from the State of New York to the Mississippi, and between Lakes Huron and Michigan, in the north, and the Ohio River and Tennessee in the south. In the Western States they are repre * De Verneuil, Bulletin, 4, 678, 1847; D. Sharpe, Quart. Journ. Geol. Soc., vol. iv. p. 145, 1847. sented by the upper part of what is termed "the Cliff Limestone." There is a grand display of this calcareous formation at the falls or rapids of the Ohio River at Louisville in Kentucky, where it much resembles a modern coral-reef. A wide extent of surface is exposed in a series of horizontal ledges, at all seasons when the water is not high; and, the softer parts of the stone having decomposed and wasted away, the harder calcareous corals stand out in relief, their erect stems sending out branches precisely as when they were living. Among other species I observed single corals, not less than 5 feet in diameter, of Favosites gothlandica, with its beautiful honeycomb structure well displayed, and, by the side of it, the Favistella, combining a similar honeycombed form with the star of the Astræa. There was also the cup-shaped Cyathophyllum, and the delicate network of the Fenestella, and that elegant and well-known European species of fossil called "the chain coral," Catenipora escharoides (see fig. 631, p. 557), with a profusion of others. These coralline forms were mingled with the joints, stems, and occasionally the heads of lily encrinites. Although hundreds of fine specimens have been detached from these rocks to enrich the museums of Europe and America, another crop is constantly working its way out, under the action of the stream, and of the sun and rain in the warm season when the channel is laid dry. The waters of the Ohio, when I visited the spot in April, 1846, were more than 40 feet below their highest level, and 20 feet above their lowest, so that large spaces of bare rock were exposed to view.* No less than 46 species of British Devonian corals are described in the monograph published in 1853 by Messrs. M. Edwards and Jules Haime (Palæontographical Society), and only six of these occur in America; a fact, observes Prof. E. Forbes, which, when we call to mind the wide latitudinal range of the Anthozoa, has an important bearing on the determination of the geography of the northern hemisphere during the Devonian epoch. We must also remember that the more conspicuous corals of these ancient reefs, viz., those which are like our cup and star corals, all belong to the Zoantharia rugosa, a sub-order which, as before stated (p. 515 et seq.), has no living representative. Hence great caution must be used in admitting all inductions drawn from the presence and forms of these zoophytes, respecting the prevalence of a warm or tropical climate in high latitudes at the time when they flourished-for such inductions, says Prof. E. Forbes, have been founded "on the mistaking of analogies for affinities." + This calcareous division also contains Goniatites, Spirifers, Pentremites, and many other genera of Mollusca and Crinoidea, corresponding to those which abound in the Devonian of Europe, and some few of the forms are the same. But the difficulty of deciding on the *Lyell's Second Visit to the United States, vol. ii. p. 277. exact parallelism of the New York subdivisions, as above enumerated, with the members of the European Devonian, is very great, so few are the species in common. This difficulty will best be appreciated by consulting the critical essay published by Mr. Hall in 1851, on the writings of European authors on this interesting question.* Indeed we are scarcely as yet able to decide on the parallelism of the principal groups even of the north and south of Scotland, or on the agreement of these again with the Devonian and Rhenish subdivisions. Canada.-In Western Canada many of the subdivisions of the New York Devonian system, as above enumerated, from the Chemung to the Oriskany formation, have been recognized by the British surveyors, and are even traceable continuously, as in the Niagara district, from the one country to the other. In Eastern Canada, or in the peninsula of Gaspé, south of the estuary of St. Lawrence, there is a great thickness of sandstone, conglomerate, and shales, referable to the Devonian period, and rich in fossil plants. The conglomerates occur in massive beds, one of them being 156 feet thick, including pebbles of white quartz, black chert, jaspers of various colors, porphyries and limestones, with a base of sandstone. They contain fragments of plants and fish-spines or Ichthyodorulites of the genera Onchus and Macharacanthum. Above these beds occur sandstones and shales of great thickness, some of the sandstones being ripple-marked. Towards the upper part of the whole series a small seam of coal has been observed with carbonaceous shale, measuring together about three inches; it rests on a bed of clay, in which are the roots of Psilophyton (see fig. 518), while stems and leaflets of the same plant are met with in the shale above the coal, and in the carbonaceous shale associated with it. At several other levels strata much like the fine clays of the Carboniferous period are penetrated vertically by the rootlets of this same Psilophyton.t South Africa.-The researches of Mr. Bain and Mr. Rubidge, at the Cape of Good Hope, have established the existence of a large Lower Devonian formation in that part of the southern hemisphere. Curiously enough, the fauna is strictly representative of that in northern regions, even to minute coincidences. The late Daniel Sharpe and Mr. Salter described many species referable to Trilobites (Homalonotus and Phacops), Annelids (Tentaculites), Mollusks (Cucullella), and large species of Crinoids allied to Rhodocrinus, &c., all of the same genera as those found in Cornwall and Germany. Vegetation of the Devonian Period. From the works of Göppert, Unger, and Bronn, we learn that the fossil plants of the Devonian rocks in Europe resemble generically, with very few exceptions, those of the coal-measures, and more ample * Report of Foster and Whitney on Geol. of L. Superior, p. 302, Washington, 1851. Sir W. E. Logan, Report of Geol. Survey of Canada, p. 394, 1863. * botanical data obtained from Canada and the United States lead to a similar conclusion respecting the flora of the same age in America. Dr. Dawson, of Montreal, in an important memoir on this subject, after enumerating thirty-two genera of Devonian plants and sixty-nine species collected in the State of New York and in Canada, observes that they belong chiefly, as in the Carboniferous period, to Gymnosperms and Cryptogams. When we peruse his catalogue of Coniferæ, Sigillaria, Calamites, Asterophyllites, Lepidodendra, Lepidostrobi, and ferns of the genera Cyclopteris, Neuropteris, Sphenopteris, &c., together with fruits, such as Cardiocarpum and Trigonocarpum, we might well suppose that we were presented with a list of carboniferous fossils; and, if told that the species differed, and that there was some admixture even of genera unknown in Europe, we might be inclined to ascribe such a want of agreement to geographical circumstances, and especially to the distance of the New from the Old World. But fortunately the coal formation is most fully developed on the other side of the Atlantic, and is singularly like that of Europe, both lithologically and in a large proportion even of the species of its fossil plants. There is also the most unequivocal evidence of relative age afforded by superposition, for the Devonian strata in the United States are seen to crop out from beneath the carboniferous on the borders of Pennsylvania and New York, where both formations are of great thickness. On comparing the species of the Middle Devonian in these countries with those of the Middle Coal-Measures, we find them all distinct, whereas some few species pass from the Upper Devonian into the Lower Carboniferous rocks. The genus most characteristic of the Devonian, and not found in the Coal, is one already alluded to, namely, Psilophyton, believed by Dr. Dawson to be a lycopodiaceous plant, branching dichotomously (see P. princeps, fig. 618 A), with stems springing from a rhizome, A b, which last has circular areoles, de, much resembling those of Stigmaria, and like it sending forth cylindrical rootlets, such as at A c. The extreme points of some of the branchlets are rolled up so as to resemble the croziers or circinate vernation of ferns, h; the leaves or bracts, i, supposed to belong to the same plant, are described by Dawson as having enclosed the fructification. The remains of Psilophyton princeps have been traced through all the members of the Devonian series in Canada and the State of New York. Some underclays in Gaspé are filled, as already stated, with its vertical rootlets just as are the fire-clays of the coal, both in Europe and America, with those of Stigmaria. One fragment of fossil wood, found some years ago by Professor Hall, in a Devonian limestone of the Hamilton group, on Lake Erie, has, according to Dawson,† the structure of an angiospermous exo * Geol. Quart. Journ., vol. xv. p. 477, 1859; also vol. xviii. p. 296, 1862. Ibid., vol. xviii. p. 305, 1862. |