CHAPTER XXVI.

OLD RED SANDSTONE, OR DEVONIAN GROUP.

Old Red Sandstone of Scotland, and borders of Wales — Fossils usually rare — "Old Red" in Forfarshire — Ichthyolites of Caithness — Distinct lithological type of Old Red in Devon and Cornwall — Term "Devonian" — Organic remains of intermediate character between those of the Carboniferous and Silurian systems — Corals and shells — Devonian strata of Westphalia, the Eifel, Russia, and the United States — Coral reef at Falls of the Ohio — Devonian flora.

It was stated in Chap. XXII. that the Carboniferous formation is surmounted by one called the "New Red," and underlaid by another called the "Old Red Sandstone."[342-C] The British strata of the last mentioned series were first recognized in Herefordshire and Scotland as of great thickness, and immediately subjacent to the coal; but they were in general so barren of organic remains, that it was difficult to find paleontological characters of sufficient importance to distinguish them as an independent group. In Scotland, and on the borders of Wales, the "Old Red" consists chiefly of red sandstone, conglomerate, and shale, with few fossils; but limestones of the same age, peculiarly rich in organic remains, were at length found in Devonshire.

I shall first advert to the characters of the group as developed in Herefordshire, Worcestershire, Shropshire, and South Wales. Its thickness has been estimated at 8000 feet, and it has been subdivided into—

1st. A quartzose conglomerate passing downwards into chocolate-red and green sandstone and marl.

2d. Cornstone and marl—red and green argillaceous spotted marls, with irregular courses of impure concretionary limestone, provincially called Cornstone.

Here, as usual, fossils are extremely rare in the clays and sandstones in which the red oxide of iron prevails; but remains of fishes of the genera Cephalaspis and Onchus have been discovered in the Cornstone.

The whole of the northern part of Scotland, from Cape Wrath to the southern flank of the Grampians, has been well described by Mr. Miller as consisting of a nucleus of granite, gneiss, and other hypogene rocks, which seem as if set in a sandstone frame.[343-A] The beds of the Old Red Sandstone constituting this frame, may once perhaps have extended continuously over the entire Grampians before the upheaval of that mountain range; for one band of the sandstone follows the course of the Moray Frith far into the interior of the great Caledonian valley; and detached hills and island-like patches occur in several parts of the interior, capping some of the higher summits in Sutherlandshire, and appearing in Morayshire like oases among the granite rocks of Strathspey. On the western coast of Ross-shire, the Old Red forms those three immense insulated hills before described ([p. 67.]), where beds of horizontal sandstone, 3000 feet high, rest unconformably on a base of gneiss, attesting the vast denudation which has taken place.

But in order to observe the uppermost part of the Old Red, we must travel south of the Grampians, and examine its junction with the bottom of the Carboniferous series in Fifeshire. This upper member may be seen in Dura Den, south of Cupar, to consist of a belt of yellow sandstone, in which Dr. Fleming first discovered scales of Holoptychius, and in which species of fish of the genera Pterichthys, Pamphractus, and others, have been met with. (For genus Pterichthys, see [fig. 400.] [p. 345.])

The beds next below the yellow sandstone are well seen in the large zone of Old Red which skirts the southern flank of the Grampians from Stonehaven to the Frith of Clyde. It there forms, together with trap, the Sidlaw Hills and the strata of the valley of Strathmore. A section of this region has been already given ([p. 48.]), extending from the foot of the Grampians in Forfarshire to the sea at Arbroath, a distance of about 20 miles, where the entire series of strata is several thousand feet thick, and may be divided into three principal masses: 1st, and uppermost, red and mottled marls, cornstone, and sandstone (Nos. 1. and 2. of the section); 2d, Conglomerate, often of vast thickness (No. 3. ibid.); 3d, Roofing and paving stone, highly micaceous, and containing a slight admixture of carbonate of lime (No. 4. ibid.). In the first of these divisions, which may be considered as succeeding the yellow sandstone of Fifeshire before mentioned, a gigantic species of fish of the genus Holoptychius has been found at Clashbinnie near Perth. Some scales (see [fig. 395.]) have been seen which measured 3 inches in length by 21/2 in breadth.

At the top of the next division, or immediately under the conglomerate (No. 3. [p. 48.]), there have been found in Forfarshire some remarkable crustaceans, with several fish of the genus named by Agassiz Cephalaspis, or "buckler-headed," from the extraordinary shield which covers the head (see [fig. 396.]), and which has often been mistaken for that of a trilobite, of the division Asaphus.

Fig. 395.

Scale of Holoptychius nobilissimus, Agas. Clashbinnie. Nat. size.

Species of the same genus are considered in England as characteristic of the second or Cornstone division ([p. 343.]).

Fig. 396.

Cephalaspis Lyellii, Agass. Length 63/4 inches. From a specimen in my collection found at Glammiss, in Forfarshire. See other figures, Agassiz, vol. ii. tab. 1. a. and 1. b.

Fig. 397.

Eggs of gasteropodous mollusk? Lower beds of Old Red, Ley's Mill, Forfarshire.

Fig. 398.

Fucoids and eggs of gasteropodous mollusk? Lower Old Red, Fife.

In the same grey paving-stones and coarse 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 spawn 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.

Fig. 399.

Fragment of spawn 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.]

Fig. 400.

Pterichthys, Agassiz; upper side, showing mouth; as restored by H. Miller.[345-A]

Beds of red shale and red sandstone, sometimes associated with pudding-stone (older than No. 3., [fig. 62.] [p. 48.]), and destitute 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 Agassiz.

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 Agassiz 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 anal 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 classification 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, assisted 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 assigned to either system, the whole taken together exhibiting a peculiar and intermediate character. But these paleontological observations alone would not have enabled us to assign, 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 substitute 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, passing into red sandstone.

The link supplied by the whole assemblage 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 shells. 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.

Fig. 401.

Favosites polymorpha, Goldf., S. Devon. From a polished specimen.

a. portion of the same, magnified to show the pores.

The Cyathophyllum cæspitosum ([fig. 402.]) and Porites pyriformis ([fig. 424.] [p. 356.]) are more peculiarly characteristic of the Devonian rocks.

In regard to the shells, 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.

Fig. 402.

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 Goniatite and Cyrtoceras, as in the Carboniferous. In some of the upper Devonian beds, a shell, resembling a flattened Goniatite, occurs, called Clymenia, by Munster (Endosiphonites, Ansted.[347-A]).

Fig. 403.

Calceola sandalina, Lam. Eifel; also South Devon.

Fig. 404.

Strygocephalus Burtini. (Terebratula porrecta, Sow.) Eifel; also South Devon.

Fig. 405.

Megalodon cucullatus, Sow. Eifel; also Bradley, S. Devon.

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.

Fig. 407.

Brontes flabellifer, Goldf. Eifel; also S. Devon.

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 Nassau. 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], associated 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 classes.

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 masses, 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 spaces 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. Römer, from the formation called "Greywacké à Posodonomyes" in the Hartz.[350-A] They are accompanied by Goniatites reticulatus Phillips, G. intercostatus Phil., and other mountain limestone species, and had been previously assigned 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.