dus, fig. 75, B), which are doubtless the bony scales of some fish resembling the modern Dog-fishes. As the above mentioned remains belong to two distinct, and at the same time highlyorganised, groups of the fishes, it is hardly likely that we are really presented here with the first examples of this great class. On the contrary, whether the so-called "Conodonts" should prove to be the teeth of fishes or not, we are justified in expecting that unequivocal remains of this group of animals will still be found in the Lower Silurian. It is interesting, also, to note that the first appearance of fishes-the lowest class of vertebrate animals—so far as known to us at present, does not take place until after all the great sub-kingdoms of invertebrates have been long in existence; and there is no reason for thinking that future discoveries will materially affect the relative order of succession thus indicated.

LITERATURE.

From the vast and daily-increasing mass of Silurian literature, it is impossible to do more than select a small number of works which have a classical and historical interest to the English-speaking geologist, or which embody researches on special groups of Silurian animals-anything like an enumeration of all the works and papers on this subject being wholly out of the question. Apart, therefore, from numerous and in many cases extremely important memoirs, by various well-known observers, both at home and abroad, the following are some of the more weighty works to which the student may refer in investigating the physical characters and succession of the Silurian strata and their fossil contents:

(1) Siluria.' Sir Roderick Murchison.

(2) Geology of Russia in Europe.' Murchison (with M. de Verneuil and Count von Keyserling).

(3) Bassin Silurien de Bohême Centrale.' Barrande.

(4) Introduction to the Catalogue of British Paleozoic Fossils in the Woodwardian Museum of Cambridge.' Sedgwick.

(5) 'Die Urwelt Russlands.' Eichwald.

(6) Report on the Geology of Londonderry, Tyrone,' &c.

Portlock.

(7) "Geology of North Wales”—Mem. Geol. Survey of Great Britain,' vol. iii. Ramsay.

(8) Geology of Canada,' 1863, Sir W. E. Logan; and the 'Reports of Progress of the Geological Survey' since 1863.

(9) Memoirs of the Geological Survey of Great Britain.'

(10) Reports of the Geological Surveys of the States of New York, Illinois, Ohio, Iowa, Michigan, Vermont, Wisconsin, Minnesota,' &c. By Emmons, Hall, Worthen, Meek, Newberry, Orton, Winchell, Dale Owen, &c.

(11) Thesaurus Siluricus.' Bigsby.

(12) British Paleozoic Fossils." M'Coy.

(13) Synopsis of the Silurian Fossils of Ireland,' (14) Appendix to the Geology of North Wales "

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vol. iii. Salter.

(15) 'Catalogue of the Cambrian and Silurian Fossils in the Woodwardian Museum of Cambridge.' Salter.

(18) Palæozoic Fossils of Canada.' Billings.

(19) 'Decades of the Geological Survey of Canada.' Billings, Salter, Rupert Jones.

(20) 'Decades of the Geological Survey of Great Britain.' Salter, Edward Forbes.

(21) 'Palæontology of New York,' vols. i.-iii.

Hall.

(22) Palæontology of Illinois.' Meek and Worthen.

(23) Palæontology of Ohio.' Meek, Hall, Whitfield, Nicholson.

(24) Silurian Fauna of West Tennessee' (Silurische Fauna des Westlichen Tennessee). Ferdinand Romer.

(25) Reports on the State Cabinet of New York.' Hall.

(26)

Lethæa Geognostica.' Bronn.

(27) Index Palæontologicus.' Bronn.

(28) Lethæa Rossica.' Eichwald.

(29) Lethæa Suecica.' Hisinger.

(30) 'Palæontologica Suecica.' Angelin.

(31) 'Petrefacta Germaniæ.' Goldfuss.

(32) Versteinerungen der Grauwacken-Formation in Sachsen.' Geinitz. (33) Organisation of Trilobites' (Ray Society). Burmeister.

(34) Monograph of the British Trilobites' (Palæontographical Society).

Salter.

(35) 'Monograph of the British Merostomata' (Palæontographical Society). Henry Woodward.

(36) Monograph of British Brachiopoda' (Palæontographical Society). Thomas Davidson.

(37) Graptolites of the Quebec Group.' James Hall.

(38) Monograph of the British Graptolitidæ.' Nicholson.

(39) Monographs on the Trilobites, Pteropods, Cephalopods, Graptolites,' &c. Extracted from the 'Système Silurien du Centre de la Bohême.' Barrande.

(40) Polypiers Fossiles des Terrains Paleozoiques,' and 'Monograph of the British Corals' (Palæontographical Society). Milne Edwards and Jules Haime.

CHAPTER XI.

THE DEVONIAN AND OLD RED SANDSTONE

PERIOD.

Between the summit of the Ludlow formation and the strata which are universally admitted to belong to the Carboniferous

series is a great system of deposits, to which the name of "Old Red Sandstone" was originally applied, to distinguish them from certain arenaceous strata which lie above the coal ("New Red Sandstone"). The Old Red Sandstone, properly so called, was originally described and investigated as occurring in Scotland and in South Wales and its borders; and similar strata occur in the south of Ireland. Subsequently it was discovered that sediments of a different mineral nature, and containing different organic remains, intervened between the Silurian and the Carboniferous rocks on the continent of Europe, and strata with similar paleontological characters to these were found occupying a considerable area in Devonshire. The name of "Devonian" was applied to these deposits; and this title, by common usage, has come to be regarded as synonymous with the name of "Old Red Sandstone." Lastly, a magnificent series of deposits, containing marine fossils, and undoubtedly equivalent to the true "Devonian of Devonshire, Rhenish Prussia, Belgium, and France, is found to intervene in North America between the summit of the Silurian and the base of the Carboniferous rocks.

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Much difficulty has been felt in correlating the true Devonian Rocks" with the typical "Old Red Sandstone”—this difficulty arising from the fact that though both formations are fossiliferous, the peculiar fossils of each have only been rarely and partially found associated together. The characteristic crustaceans and many of the characteristic fishes of the Old Red are wanting in the Devonian; whilst the corals_and marine shells of the latter do not occur in the former. It is impossible here to enter into any discussion as to the merits of the controversy to which this difficulty has given origin. No one, however, can doubt the importance and reality of the Devonian series as an independent system of rocks to be intercalated in point of time between the Silurian and the Carboniferous. The want of agreement, both lithologically and palæontologically, between the Devonian and the Old Red, can be explained by supposing that these two formations, though wholly or in great part contemporaneous, and therefore strict equivalents, represent deposits in two different geographical areas, laid down under different conditions. On this view, the typical Devonian rocks of Europe, Britain, and North America are the deep-sea deposits of the Devonian period, or, at any rate, are genuine marine sediments formed far from land. On the other hand, the “Old Red Sandstone" of Britain and the corresponding "Gaspé Group" of Eastern

Canada represent the shallow-water shore-deposits of the same period. In fact, the former of these last - mentioned deposits contains no fossils which can be asserted positively to be marine (unless the Eurypterids be considered so); and it is even conceivable that it represents the sediments of an inland sea. Accepting this explanation in the meanwhile, we may very briefly consider the general succession of the deposits of this period in Scotland, in Devonshire, and in North America.

In Scotland the "Old Red" forms a great series of arenaceous and conglomeratic strata, attaining a thickness of many thousands of feet, and divisible into three groups. Of these, the Lower Old Red Sandstone reposes with perfect conformity upon the highest beds of the Upper Silurian, the two formations being almost inseparably united by an intermediate series of "passage-beds." In mineral nature this group consists principally of massive conglomerates, sandstones, shales, and concretionary limestones; and its fossils consist chiefly of large crustaceans belonging to the family of the Eurypterids, fishes, and plants. The Middle Old Red Sandstone consists of flagstones, bituminous shales, and conglomerates, sometimes with irregular calcareous bands; and its fossils are principally fishes and plants. It may be wholly wanting, when the Upper Old Red seems to repose unconformably upon the lower division of the series. The Upper Old Red Sandstone consists of conglomerates and grits, along with a great series of red and yellow sandstones-the fossils, as before, being fishes and remains of plants. The Upper Old Red graduates upwards conformably into the Carboniferous series.

The Devonian rocks of Devonshire are likewise divisible into a lower, middle, and upper division. The Lower Devonian or Lynton Group consists of red and purple sandstones, with marine fossils, corresponding to the "Spirifer Sandstein" of Germany, and to the arenaceous deposits (Schoharie and Cauda-Galli Grits) at the base of the American Devonian. The Middle Devonian or Ilfracombe Group consists of sandstones and flags, with calcareous slates and crystalline limestones, containing many corals. It corresponds with the great "Eifel Limestone" of the Continent, and, in a general way, with the Corniferous Limestone and Hamilton group of North America. The Upper Devonian or Pilton Group, lastly, consists of sandstones and calcareous shales which correspond with the 66 Clymenia Limestone" and " Cypridina Shales” of the Continent, and with the Chemung and Portage groups of

North America. It seems quite possible, also, that the socalled "Carboniferous Slates" of Ireland correspond with this group, and that the former would be more properly regarded as forming the summit of the Devonian than the base of the Carboniferous.

In no country in the world, probably, is there a finer or more complete exposition of the strata intervening between the Silurian and Carboniferous deposits than in the United States. The following are the main subdivisions of the Devonian rocks in the State of New York, where the series may be regarded as being typically developed (fig. 67):

(1) Cauda-Galli Grit and Schoharie Grit.-Considering the "Oriskany Sandstone" as the summit of the Upper Silurian, the base of the Devonian is constituted by the arenaceous deposits known by the above names, which rest quite conformably upon the Silurian, and which represent the Lower Devonian of Devonshire. The Cauda-Galli Grit is so called from the abundance of a peculiar spiral fossil (Spirophyton cauda-Galli), which is of common occurrence in the Carboniferous rocks of Britain, and is supposed to be the remains of a sea-weed.

(2) The Corniferous or Upper Helderberg Limestone. — A series of limestones usually charged with considerable quantities of siliceous matter in the shape of hornstone or chert (Lat. cornu, horn). The thickness of this group rarely exceeds 300 feet; but it is replete with fossils, more especially with the remains of corals. The Corniferous Limestone is the equivalent of the coral-bearing limestones of the Middle Devonian of Devonshire and the great "Eifel Limestone" of Germany.

(3) The Hamilton Group-consisting of shales at the base ("Marcellus shales "); flags, shales, and impure limestones ("Hamilton beds") in the middle; and again a series of shales ("Genesee Slates") at the top. The thickness of this group varies from 200 to 1200 feet, and it is richly charged with marine fossils.

(4) The Portage Group.—A great series of shales, flags, and shaly sandstones, with few fossils.

(5) The Chemung Group.-Another great series of sandstones and shales, but with many fossils. The Portage and Chemung groups may be regarded as corresponding with the Upper Devonian of Devonshire. The Chemung beds are succeeded by a great series of red sandstones and shales-the