A Manual of Elementary Geology Part 52

In the same grey paving-stones and coa.r.s.e roofing-slates, in which the _Cephalaspis_ occurs, in Forfarshire and Kincardineshire, the remains of marine plants or fucoids abound. They are frequently accompanied by groups of hexagonal, or nearly hexagonal markings, which consist of small flattened carbonaceous bodies, placed in a slight depression of the sandstone or shale. (See figs. 397 and 398.) They much resemble in form the sp.a.w.n of the recent Natica (see fig. 399.), in which the eggs are arranged in a thin layer of sand, and seem to have acquired a polygonal form by pressing against each other. The substance of the egg, if fossilized, might give rise to small pellicles of carbonaceous matter.

[Ill.u.s.tration: Fig. 399. Fragment of sp.a.w.n of British species of _Natica_.]

These fossils I have met with, both to the north of Strathmore, in the vertical shale beneath the conglomerate, and in the same beds in the Sidlaw hills, at all the points where fig. 4. is introduced in the section, p. 48.

[Ill.u.s.tration: Fig. 400. _Pterichthys_, Aga.s.siz; upper side, showing mouth; as restored by H. Miller.[345-A]]

Beds of red shale and red sandstone, sometimes a.s.sociated with pudding-stone (older than No. 3., fig. 62. p. 48.), and dest.i.tute of organic remains, separate, in the region of Strathmore, the above-described fossiliferous strata from the older crystalline rocks of the Grampians.

But, in the north of Scotland, we find, at the base of the Old Red, other grey slaty sandstones, in the counties of Banff, Nairn, Moray, Cromarty, Caithness, and in Orkney, rich in ichthyolites of peculiar forms, belonging to the genera _Pterichthys_ (fig. 400.), _Coccosteus_, _Diplopterus_, _Dipterus_, _Cheiracanthus_, and others of Aga.s.siz.

Five species of _Pterichthys_ have been found in this lowest division of the Old Red. The wing-like appendages, whence the genus is named, were first supposed by Mr. Miller to be paddles, like those of the turtle; but Aga.s.siz regards them as weapons of defence, like the occipital spines of the River Bull-head (_Cottus gobio_, Linn.); and considers the tail to have been the only organ of motion. The genera _Dipterus_ and _Diplopterus_ are so named, because their two dorsal fins are so placed as to front the a.n.a.l and ventral fins, so as to appear like two pairs of wings. They have bony enamelled scales.

_South Devon and Cornwall._--A great step was made in the cla.s.sification of the slaty and calciferous strata of South Devon and Cornwall in 1837, when a large portion of the beds, previously referred to the "transition" or most ancient fossiliferous series, were found to belong in reality to the period of the Old Red Sandstone. For this reform we are indebted to the labours of Professor Sedgwick and Sir R. Murchison, a.s.sisted by a suggestion of Mr. Lonsdale, who, in 1837, after examining the South Devonshire fossils, perceived that some of them agreed with those of the Carboniferous group, others with those of the Silurian, while many could not be a.s.signed to either system, the whole taken together exhibiting a peculiar and intermediate character. But these paleontological observations alone would not have enabled us to a.s.sign, with accuracy, the true place in the geological series of these slate-rocks and limestones of South Devon, had not Messrs. Sedgwick and Murchison, in 1836 and 1837, discovered that the culmiferous or anthracitic shales of North Devon belonged to the Coal, and not, as preceding observers had imagined, to the transition period.

As the strata of South Devon here alluded to are far richer in organic remains than the red sandstones of contemporaneous date in Herefordshire and Scotland, the new name of the "Devonian system" was proposed as a subst.i.tute for that of Old Red Sandstone.

The rocks of this group in South Devon consist, in great part, of green chloritic slates, alternating with hard quartzose slates and sandstones.

Here and there calcareous slates are interstratified with blue crystalline limestone, and in some divisions conglomerates, pa.s.sing into red sandstone.

The link supplied by the whole a.s.semblage of imbedded fossils, connecting as it does the paleontology of the Silurian and Carboniferous groups, is one of the highest interest, and equally striking, whether we regard the _genera_ of corals or of sh.e.l.ls. The _species_ are almost all distinct.

Among the more abundant corals, we find the genera _Favosites_ and _Cyathophyllum_, common on the one hand to the Mountain limestone, and on the other to the Silurian system. Some few even of the _species_ are common to the Devonian and Silurian groups, as, for example, _Favosites polymorpha_ (fig. 401.), very abundant in South Devon.

[Ill.u.s.tration: Fig. 401. _Favosites polymorpha_, Goldf., S. Devon.

From a polished specimen.

_a._ portion of the same, magnified to show the pores.]

The _Cyathophyllum caespitosum_ (fig. 402.) and _Porites pyriformis_ (fig.

424. p. 356.) are more peculiarly characteristic of the Devonian rocks.

In regard to the sh.e.l.ls, all the brachiopodous genera, such as _Terebratula_, _Orthis_, _Spirifer_, _Atrypa_, and _Productus_, which are found in the Mountain limestone, occur, together with those of the Silurian system, except the _Pentamerus_. Some forms, however, seem exclusively Devonian, as for example, _Calceola sandalina_ (fig. 403.) and _Strygocephalus Burtini_ (fig. 404.), which have been met with both in the Eifel, in Germany, and in Devonshire, in the very lowest Devonian beds.

[Ill.u.s.tration: Fig. 402. Cyathophyllum.

_a._ _Cyathophyllum caespitosum_, Goldf., Plymouth.

_b._ a terminal star.

_c._ vertical section exhibiting transverse plates, and part of another branch.]

Among the peculiar lamellibranchiate bivalves, also common to Devonshire and the Eifel, we find _Megalodon cucullatus_ (fig. 405.). Several spiral univalves are abundant, among which are many species of _Pleurotomaria_ and _Euomphalus_. Among the Cephalopoda we find _Bellerophon_ and _Orthoceras_, as in the Silurian and Carboniferous groups, and _Goniat.i.te_ and _Cyrtoceras_, as in the Carboniferous. In some of the upper Devonian beds, a sh.e.l.l, resembling a flattened _Goniat.i.te_, occurs, called _Clymenia_, by Munster (_Endosiphonites_, Ansted.[347-A]).

[Ill.u.s.tration: Fig. 403. _Calceola sandalina_, Lam. Eifel; also South Devon.

_a._ both valves united.

_b._ inner side of opercular valve.]

[Ill.u.s.tration: Fig. 404. _Strygocephalus Burtini_. (_Terebratula porrecta_, Sow.) Eifel; also South Devon.

_a._ valves united.

_b_. side view of same.

_c._ interior of larger valve, showing thick part.i.tion, and thinner one continued from it.]

[Ill.u.s.tration: Fig. 405. _Megalodon cucullatus_, Sow. Eifel; also Bradley, S. Devon.

_a._ the valves united.

_b._ interior of valve, showing the large cardinal tooth.]

[Ill.u.s.tration: Fig. 406. _Clymenia linearis_, Munster. (_Endosiphonites carinatus_, Ansted.) Cornwall.]

A peculiar species of trilobite, called _Brontes flabellifer_ (fig. 407.), is found in the Devonian strata of the Eifel and in South Devon. It should be observed, however, that the head in the specimen here figured by Goldfuss, the most perfect which could be obtained, is incomplete, and a restoration has been attempted by Mr. Salter in fig. 408., from data supplied by other species of the same genus occurring in older rocks.

[Ill.u.s.tration: Fig. 407. _Brontes flabellifer_, Goldf. Eifel; also S. Devon.]

[Ill.u.s.tration: Fig. 408. Restored outline of head of _Brontes flabellifer_.]

For determining the true equivalents of the Devonian group in the Rhenish provinces and adjacent parts of Germany, we are indebted to the labours of Messrs. Sedgwick and Murchison, in 1839, from which it appears that rocks of that age emerge from beneath the coal-field of Westphalia, and are also found in troughs among the Silurian rocks in Na.s.sau. Many of the limestones, particularly those on the river Lahn, are identical, both in structure and in coralline remains, with the beautiful marbles of Babbacombe, Torquay, and Plymouth.

The limestones of the Eifel, long ago celebrated for their fossils, and which lie in a basin supported by Silurian rocks, are found to be referable to the lower part of the Devonian system.

In Russia, also, Messrs. Murchison and De Verneuil have shown (1840) that the "Old Red" group occupies a wide area south from St. Petersburg. It was formerly supposed to be the New Red Sandstone, on account of its saliferous and gypseous beds; but it is now proved to be the Old Red by containing ichthyolites of genera which characterize this group in the British Isles, as, for example, _Holoptychius_, _Coccosteus_, _Diplopterus_, &c.[349-A], a.s.sociated with mollusca found in the Devonian of Western Europe. Among the fish are also many species of sharks of the Cestraciont division, a fact worthy of notice, because the squaloid fishes of the present day offer the highest organization of the brain and of the generative organs, and make, in these respects, the nearest approach to the higher vertebrate cla.s.ses.

_Devonian Strata in the United States._

The position of this formation between the carboniferous rocks of Pennsylvania and Ohio, is pointed out in the section, fig. 379. p. 327., and it is a remark of M. de Verneuil that in no European country is there so complete and uninterrupted a development of the Devonian system as in North America. At the falls of the Ohio, at Louisville, in Kentucky, there is a grand display of one of the limestones of this period, resembling 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, and many of them send out branches from their erect stems precisely as if they were living. Among other species I observed large ma.s.ses, 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 _Astrea_. 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_, with a profusion of others (see fig. 423. p. 355.). 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 s.p.a.ces of bare rock were exposed to view.[349-B]

_Devonian Flora._

With the exception of the fucoids above mentioned (p. 344.), but little is known with certainty of the plants of the Devonian group. Those found in the department of La Sarthe in France, and in various parts of Brittany, formerly referred to the Devonian era, have been shown (in 1850), by M. de Verneuil, to belong to the carboniferous series. The same may be said of the species of _Lepidodendron_, _Knorria_, _Calamite_, _Sagenaria_, and other genera recently figured (1850), by Mr. F. A. Romer, from the formation called "Greywacke a Posodonomyes" in the Hartz.[350-A] They are accompanied by _Goniat.i.tes reticulatus_ Phillips, _G. intercostatus_ Phil., and other mountain limestone species, and had been previously a.s.signed to the oldest part of the carboniferous series by Messrs. Murchison and Sedgwick.

If hereafter we should become well acquainted with the land plants of the Devonian era, we may confidently expect that nearly all of them will agree generically with those of the carboniferous period, but the species will be as different as are the Devonian vertebrate and invertebrate animals from the fossil species of the Coal.

FOOTNOTES:

[342-C] See section, fig. 318. p. 287.

[343-A] The Old Red Sandstone, by Hugh Miller, 1841.

[345-A] Old Red Sandstone. Plate 1. fig. 1. Mr. M.'s description of the fish is most graphic and correct.

[347-A] Camb. Phil. Trans., vol. vi. pl. 8. fig. 2.

[349-A] See Proceedings of Geol. Soc., and the anniversary speech of Dr. Buckland, P. G. S., for 1841.

[349-B] Lyell's Second Visit to the United States, vol. ii. p. 277.