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

by Sir Charles Lyell

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.