The course of creation

by John Anderson

CHAPTER II.
THE PERMIAN SYSTEM—NEW RED SANDSTONE.

The geological formations described are succeeded in the ascending order by the Permian system of deposits—a term borrowed from the department of Perm in Russia, where the strata cover an area about twice the size of France. This constitutes the new red sandstone of English geology, and has many equivalents in other countries. Thus, in the lower division of the group or true Permian, there occurs the Zechstein and Lower Bunter series of strata: in the upper or triassic division, the equivalents are the Upper Bunter and Grés Bigarré, or variegated sandstone, the Muschelkalk and Keuper of French and German authors. The system is largely developed in America, Africa, India and China; where, as in Britain, the deposits are of extremely varied mineral characters, consisting of grits, sandstones, marls, limestones, gypsum, and rock-salt, each presenting its own family types of vegetable and animal life.

The new red sandstone extends across England without interruption, through the medial or central counties, and ranges nearly north-east by south-west. The two great divisions of which it consists are everywhere well marked, the dolomitic or magnesian limestone forming the base, and giving character to the lower permian group; the upper triassic group is sufficiently distinguished by the rock-salt deposit which is wholly included in this part of the formation. Each, too, has its own peculiar set of fossils. Those of the former are allied to animals that flourished during the carboniferous period; two genera of fishes, the palæoniscus and pylopterus, are common to both. The fauna and flora of the triassic group are regarded as entirely new, neither borrowing from nor imparting anything to illustrate the organisms of the older families of rocks. The one series of strata thus represents the coming of a new, the other records the departure of a past state of things.

In the central counties of England this deposit expands into a great plain, surrounded on all sides by the coal-measures, while within its own area several basins—as those of Leicester, Warwick, and South Stafford—are included, being completely isolated by the new red. An interesting economic question hence arises—Do the coal minerals occupy the whole or any considerable portion, of the extensive area covered by this formation? An equally important geological problem is connected with the solution of the question—namely, What are the general relations of the older to the newer deposits of the district? The researches of Sir R. I. Murchison, and more recently, of the geological survey, have shown that the three groups of stratified rocks in South Staffordshire, the new red sandstone, coal-measures, and silurian beds, are each unconformable to the other—that the upper rests indifferently upon the two lower formations—and that where the old red occurs, the new is sometimes in immediate contact. It is inferred from this, that there was an uplifting of the silurian rocks, along with considerable denudation, previous to the deposition of the carboniferous strata. Mr. Jukes has observed pebbles of coal, in great abundance, in the lower beds of the new red sandstone, and thence deduces the following conclusions:—1. That there was a movement and denudation of the coal-measures, amounting, in some localities, to their entire destruction and removal, before the deposition of the new red sandstone. 2. That, subsequently to the deposition of the new red sandstone, there was a very great movement of all these rocks, producing their present faults and inclined positions. 3. That the boundaries of the South Staffordshire coal-field, as far as examined, present examples of three kinds of relation between the coal-measures and new red sandstones; i. e., by conformable succession; by fault, the coal-measures being present on the downcast side; and, thirdly, where the destruction of the coal-measures has brought the new red sandstone into immediate contact with the silurian strata. The author of this paper is farther of opinion that, while there is a great probability that the larger part of the new red sandstone plain conceals productive coal-measures, there is the presumption that these will not be found at a depth of less than 500 or 600 yards below the surface.

Corresponding with these views it will be remembered that, after the deposition of the coal-measures, there succeeded a period of violent plutonic action, whereby the formation was dislocated, broken up into smaller sections or basins, and pierced by the igneous rocks. There would, consequently, during this season of paroxysm, be a vast destruction of animal and vegetable life. The indurated crust would everywhere undergo great trituration. Gravel, sand, and mud of every quality would be cast along the shores, or silted up in the deeper hollows. And then again would come a term of general repose, as the angry elements subsided, exhausted by their own violence. The scene was actually or nearly as described; and, in the aspect of the older denuded and uplifted rocks, as above represented, there are the most striking evidences of the agitations of the period. The exuberant flora of the carboniferous age, suffered prodigiously, or became utterly extinct. Conglomerates were formed which exhibit few traces of organic life. To these succeeded the vast areas of the fine-grained sandstones, and gypseous and saliferous marls, everywhere nearly horizontal, and still undisturbed on their ancient beds. The tribes of animals were abridged in numbers, changed or modified in structure, so as to suit the altered state of things. The rain-drop, ripple-mark, and foot-print are all witnesses to be adduced of the mighty change, as they are all proofs of the doings of Him who holds the waters in the hollow of his hand, makes the clouds to distill in showers, issues his command to the hurricane and the earthquake, and restores in renovated beauty the face of nature.

The lowest bed of the formation is the magnesian limestone, which derives its name from the quantity of carbonate of magnesia distributed through the matter of the rock, amounting in some instances to as much as sixty per cent. It is likewise called the dolomitic limestone, from M. Dolomieu, who first investigated its granular crystalline structure. This limestone is generally of a yellow color, glimmering luster, passing occasionally into blue and brick-red varieties, and exfoliates in thin plates, or breaks up in large botryoidal masses. In this form it occurs at the cliffs of Durham, where it assumes the grouping and arrangement of chain-shot; and, as the beds are distinctly stratified, the face of the rock has a very striking and pleasing effect. In the more southern counties, this formation exists generally in the form of a conglomerate, supposed to be derived from the debris of the older carboniferous limestone united by a dolomitic paste; thus illustrating the source and mode of the deposit, while in the organic remains there has been traced a regular gradation between the types of the older sub-carboniferous and the successive newer strata.

Let us consider some of the more remarkable forms, tracings, and ingredients of the formation.

I. The Organic Remains are scanty as compared with those of the age immediately anterior. The vegetable forms, as yet detected, are new and distinct. The fishes consist of six or seven genera, and about as many different species. And here commences, it is supposed, the singular change in their ossification, for which science can assign no reason, as it cannot detect the least appearance of graduation into the new, for the first time, begun and completed change. The fishes of the formation present the Homocercal—that is, the equally-lobed, or one-lobed tail-fin,—a structure peculiar to existing races, with the exception of the shark, sturgeon, and a few others, and form a striking contrast to those in the antecedent groups, which were all possessed of the Heterocercal, or unequally-lobed tail-fin.

Heterocercal. Homocercal.