(11) 'Thesaurus Siluricus.' Bigsby.

(12) 'British Palaeozoic Fossils.' M'Coy.

(13) 'Synopsis of the Silurian Fossils of Ireland,' M'Coy.

(14) "Appendix to the Geology of North Wales"--'Mem. Geol. Survey,'

vol. iii. Salter.

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

(16) 'Characteristic British Fossils.' Baily.

(17) 'Catalogue of British Fossils.' Morris.

(18) 'Palaeozoic 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) 'Palaeontology of New York,' vols. i.-iii. Hall.

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

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

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

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

(26) 'Lethaea Geognostica.' Bronn.

(27) 'Index Palaeontologicus.' Bronn.

(28) 'Lethaea Rossica.' Eichwald.

(29) 'Lethaea Suecica.' Hisinger.

(30) 'Palaeontologica Suecica.' Angelin.

(31) 'Petrefacta Germaniae.' Goldfuss.

(32) 'Versteinerungen der Grauwacken-Formation in Sachsen.' Geinitz.

(33) 'Organisation of Trilobites' (Ray Society). Burmeister.

(34) 'Monograph of the British Trilobites' (Palaeontographical Society). Salter.

(35) 'Monograph of the British Merostomata' (Palaeontographical Society).

Henry Woodward.

(36) 'Monograph of British Brachiopoda' (Palaeontographical Society).

Thomas Davidson.

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

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

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

(40) 'Polypiers Fossiles des Terrains Paleozoiques,' and 'Monograph of the British Corals' (Palaeontographical 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 palaeontological characters to these were found occupying a considerable area in Devonshire.

The name of "Devonian" was applied to these deposits; and this t.i.tle, 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.

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 a.s.sociated together. The characteristic crustaceans and many of the characteristic fishes of the Old Red are wanting in the Devonian; whilst the corals and marine sh.e.l.ls 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 palaeontologically, 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 "Gaspe Group" of Eastern Canada represent the shallow-water sh.o.r.e-deposits of the same period. In fact, the former of these last-mentioned deposits contains no fossils which can be a.s.serted 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 "pa.s.sage-beds."

In mineral nature this group consists princ.i.p.ally of ma.s.sive 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 princ.i.p.ally 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 "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 so-called "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 const.i.tuted 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 quant.i.ties 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 "Catskill Group"--which pa.s.s conformably upwards into the Carboniferous, and which may perhaps be regarded as the equivalent of the great sandstones of the Upper Old Red in Scotland.

Throughout the entire series of Devonian deposits in North America no unconformability or physical break of any kind has. .h.i.therto been detected; nor is there any marked interruption to the current of life, though each subdivision of the series has its own fossils.

No completely natural line can thus be indicated, dividing the Devonian in this region from the Silurian on the one hand, and the Carboniferous on the other hand. At the same time, there is the most ample evidence, both stratigraphical and palaeontological, as to the complete independence of the American Devonian series as a distinct life-system between the older Silurian and the later Carboniferous. The subjoined section (fig. 76) shows diagrammatically the general succession of the Devonian rocks of North America.

[Ill.u.s.tration: Fig. 76. GENERALIZED SECTION OF THE DEVONIAN ROCKS OF NORTH AMERICA.]

[Ill.u.s.tration: Fig. 77.--Restoration of _Psilophyton princeps_.

Devonian, Canada. (After Dawson.)]

As regards the _life_ of the Devonian period, we are now acquainted with a large and abundant terrestrial _flora_--this being the first time that we have met with a land vegetation capable of reconstruction in any fulness. By the researches of Goeppert, Unger, Dawson, Carruthers, and other botanists, a knowledge has been acquired of a large number of Devonian plants, only a few of which can be noticed here. As might have been antic.i.p.ated, the greater number of the vegetable remains of this period have been obtained from such shallow-water deposits as the Old Red Sandstone proper and the Gaspe series of North America, and few traces of plant-life occur in the strictly marine sediments.

Apart from numerous remains, mostly of a problematical nature, referred to the comprehensive group of the Sea-weeds, a large number of Ferns have now been recognised, some being, of the ordinary plant-like type (_Pecopteris, Neuropteris, Alethopteris, Sphenopteris_, &c.), whilst others belong to the gigantic group of the "Tree-ferns" (_Psaronius, Caulopteris_, &c.) Besides these there is an abundant development of the singular extinct types of the _Lepidodendroids_, the _Sigillarioids_, and the _Calamites_, all of which attained their maximum in the Carboniferous. Of these, the _Lepidodendra_ may be regarded as gigantic, tree-like Club-mosses (_Lycopodiaceoe_); the _Calamites_ are equally gigantic Horse-tails (_Equisetaceoe_); and the _Sigillarioids_, equally huge in size, in some respects hold a position intermediate between the Club-mosses and the Pines (Conifers). The Devonian rocks have also yielded traces of many other plants (such as _Annularia, Asterophyllites, Cardiocarpon_, &c.), which acquire a greater pre-dominance in the Carboniferous period, and which will be spoken of in discussing the structure of the plants of the Coal-measures. Upon the whole, the one plant which may be considered as specially characteristic of the Devonian (though not confined to this series) is the _Psilophyton_ (fig. 77) of Dr Dawson.

These singular plants have slender branching stems, with spa.r.s.e needle-shaped leaves, the young stems being at first coiled up, crosier-fashion, like the young fronds of ferns, whilst the old branches carry numerous spore-cases. The stems and branches seem to have attained a height of two or three feet; and they sprang from prostrate "root-stocks" or creeping stems. Upon the whole, Princ.i.p.al Dawson is disposed to regard _Psilophyton_ as a "generalised type" of plants intermediate between the Ferns and the Club-mosses. Lastly, the Devonian deposits have yielded the remains of the first actual _trees_ with which we are as yet acquainted. About the nature of some of these (_Ormoxylon_ and _Dadoxylon_) no doubt can be entertained, since their trunks not only show the concentric rings of growth characteristic of exogenous trees in general, but their woody tissue exhibits under the microscope the "discs" which are characteristic of the wood of the Pines and Firs (see fig. 2). The singular genus _Prototaxites_, however, which occurs in an older portion of the Devonian series than the above, is not in an absolutely unchallenged position.

By Princ.i.p.al Dawson it is regarded as the trunk of an ancient _Conifer_--the most ancient known; but Mr Carruthers regards it as more probably the stem of a gigantic sea-weed. The trunks of _Prototaxites_ (fig. 78, A) vary from one to three feet in diameter, and exhibit concentric rings of growth; but its woody fibres have not hitherto been clearly demonstrated to possess discs.

Before leaving the Devonian vegetation, it may be mentioned that the hornstone or chert so abundant in the Corniferous limestone of North America has been shown to contain the remains of various microscopic plants (_Diatoms_ and _Desmids_). We find also in the same siliceous material the singular spherical bodies, with radiating spines, which occur so abundantly in the chalk flints, and which are termed _Xanthidia_. These may be regarded as probably the spore-cases of the minute plants known as _Desmidioe_.

[Ill.u.s.tration: Fig. 78.--A, Trunk of _Prototaxites Logani_, eighteen inches in diameter, as seen in the cliff near L'Anse Brehaut, Gaspe; B, Two wood-cells showing spiral fibres and obscure pores, highly magnified. Lower Devonian, Canada. (After Dawson)]

The Devonian _Protozoans_ have still to be fully investigated.

True Sponges (such as _Astrtoeospongia, Sph.o.e.rospongia_, &c.) are not unknown; but by far the commonest representatives of this sub-kingdom in the Devonian strata are _Stromatopora_ and its allies. These singular organisms (fig. 79) are not only very abundant in some of the Devonian limestones--both in the Old World and the New--but they often attain very large dimensions. However much they may differ in minor details, the general structure of these bodies is that of numerous, concentrically-arranged, thin, calcareous laminae, separated by narrow inters.p.a.ces, which in turn are crossed by numerous delicate vertical pillars, giving the whole ma.s.s a cellular structure, and dividing it into innumerable minute quadrangular compartments. Many of the Devonian _Stromatoporoe_ also exhibit on their surface the rounded openings of ca.n.a.ls, which can hardly have served any other purpose than that of permitting the sea-water to gain ready access to every part of the organism.

[Ill.u.s.tration: Fig. 79.--a, Part of the under surface of _Stromatopora tuberculata_, showing the wrinkled bas.e.m.e.nt membrane and the openings of water-ca.n.a.ls, of the natural size; b, Portion of the upper surface of the same, enlarged; c, Vertical section of a fragment, magnified to show the internal structure. Corniferous Limestone, Canada. (Original.)]

[Ill.u.s.tration: Fig. 80.--_Cystiphyllum vesiculosum_, showing a succession of cups produces by budding from the original coral.

Of the natural size. Devonian, America and Europe. (Original.)]

[Ill.u.s.tration: Fig. 81--_Zaphrentis cornicula_, of the natural size. Devonian, America. (Original.)]

[Ill.u.s.tration: Fig. 82--_Heliophyllum exiguum_, viewed from in front and behind. Of the natural size. Devonian, Canada. (Original.)]

[Ill.u.s.tration: Fig. 83.--Portion of a ma.s.s of _Crepidophyllum Archiaci_, of the natural size. Hamilton Formation, Canada. (After Billings.)]

No true _Graptolites_ have ever been detected in strata of Devonian age; and the whole of this group has become extinguished--unless we refer here the still surviving _Dictyonemoe_. The _Coelenterates_, however, are represented by a vast number of _Corals_, of beautiful forms and very varied types. The marbles of Devonshire, the Devonian limestones of the Eifel and of France, and the calcareous strata of the Corniferous and Hamilton groups of America, are often replete with the skeletons of these organisms--so much so as to sometimes ent.i.tle the rock to be considered as representing an ancient coral-reef. In some instances the Corals have preserved their primitive calcareous composition; and if they are embedded in soft shales, they may weather out of the rock in almost all their original perfection. In other cases, as in the marbles of Devonshire, the matrix is so compact and crystalline that the included corals can only be satisfactorily studied by means of polished sections. In other cases, again, the corals have been more or less completely converted into flint, as in the Corniferous limestone of North America. When this is the case, they often come, by the action of the weather, to stand out from the enclosing rock in the boldest relief, exhibiting to the observer the most minute details of their organization. As before, the princ.i.p.al representatives of the Corals are still referable to the groups of the _Rugosa_ and _Tabulata_. Amongst the Rugose group we find a vast number of simple "cup-corals," generally known by the quarrymen as "horns," from their shape. Of the many forms of these, the species of _Cyathophyllum, Heliophyllum_ (fig. 82), _Zaphrentis_ (fig. 81), and _Cystiphyllum_ (fig. 80), are perhaps those most abundantly represented--none of these genera, however, except _Heliophyllum_, being peculiar to the Devonian period. There are also numerous compound Rugose corals, such as species of _Eridophyllum, Diphyphyllum, Syringopora, Phillipsastroea_, and some of the forms of _Cyathophyllum_ and _Crepidophyllum_ (fig. 83). Some of these compound corals attain a very large size, and form of themselves regular beds, which have an a.n.a.logy, at any rate, with existing coral-reefs, though there are grounds for believing that these ancient types differed from the modern reef-builders in being inhabitants of deep water.

The "Tabulate Corals" are hardly less abundant in the Devonian rocks than the _Rugosa_; and being invariably compound, they hardly yield to the latter in the dimensions of the aggregations which they sometimes form.

[Ill.u.s.tration: Fig. 84.--Portion of a ma.s.s of _Favosites Gothlandica_, of the natural size. Upper Silurian and Devonian of Europe and America. (Original.) Billings.]