A Manual of Elementary Geology / or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

by Sir Charles Lyell

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.])