The Principles of Stratigraphical Geology

by J. E. Marr

The ridges separate different deposits of Devonian rocks, which were possibly deposited in isolated areas, though there was probably connexion between them at any rate at times.

The Old Red Sandstone type consists to a large extent, as the name implies, of sandstones which are coloured red by a deposit of peroxide of iron around the sand grains. They are separable into a lower and upper division with an unconformity often occurring between them. The lower Old Red passes down in places into the Silurian rocks with perfect conformity, and the upper Old Red similarly passes up into the Carboniferous strata. The existence of pebble beds at different horizons is a noteworthy feature. They are frequently found at or near the base of the two divisions. The sandstones of the lower division are often accompanied by flagstones, while the red sandstones of the upper division usually have deposits of yellow and brown sandstone intercalated between them. Inconstant beds of limestone, known as cornstones, are found in both divisions, and Prof. Sollas has shown that some of these, at any rate, are true mechanical deposits, formed by the destruction of pre-existing strata of limestone and the deposition of the resulting fragments from a state of suspension. In Scotland a great thickness of volcanic material of various kinds is associated with the two divisions. For the sake of simplicity this is omitted from [Fig. 19][85]. It is not known how far normal sediments are associated with the Old Red Sandstone type of deposit. The existence of some in South Wales is suggested by evidence supplied by the late Mr J. W. Salter.

[85] For an account of these and all other British volcanic rocks the reader is referred to Sir A. Geikie's work on The Ancient Volcanoes of Great Britain. Macmillan and Co., 1897.

The Devon type, as will be seen in the figure, consists of rocks which are to a great extent of normal character. We find in Devonshire alternations of sandstones, shales and limestones, but even here, red sandstones, which are comparable with those of the Old Red type occur in diminished amount: the Foreland Grits and Pickwell Down Sandstones are both coloured red, and are like the sandstones formed further north. The recognition of this fact induces one to believe that the contrast between the two types of rock which are found at a short distance from one another on opposite sides of the Bristol Channel is not so marked as one is sometimes led to suppose.

The rocks of North Devon differ from those of South Devon chiefly owing to the amount of calcareous sediment found in the two areas, for limestones occur in South Devon to a great extent, and in North Devon there is a comparative poverty of this kind of sediment. Here, again, the apparent difference is possibly greater than the real one. The North Devon limestones have in places been stretched out after their formation and thus rendered thinner, and the highly-cleaved limestones are occasionally mistaken for shales, while in South Devon there is evidence of thickening of the limestones by folding subsequently to their deposition. Allowing for these changes, however, there is still a marked diminution in the amount of coarse mechanical sediments and increase in the quantity of calcareous matter as one passes from North to South Devon, and this prepares one for the condition of things met with on parts of the continent, where the mechanical sediments become finer and thinner on the whole as one travels southward, until, when we reach the Bohemian area, the Devonian rocks are found to be largely composed of calcareous sediments.

It is interesting to find that in North America the two types of Devonian strata recur, and present characters generally similar to those which they possess upon this side of the Atlantic.

Passing now to a consideration of the conditions under which the Devonian rocks were deposited, we may examine the bearing of the character of the strata as a whole, and then proceed to more detailed consideration of the nature and conditions of deposits of the two types.

The gradual increase in calcareous matter and dying out of mechanical sediments as one travels southward points to recession from land in that direction, and we have already seen that the epeirogenic and orogenic movements of this continental period elevated the Silurian sea-floor in the north, and gave rise to a Northern Continent, while oceanic conditions continued further South, and allowed the accumulation of sediments lying conformably upon those of Silurian age, and giving indications of the prevalence of physical conditions during Devonian times which were in the main similar to those of the preceding Silurian period.

In the shallow waters adjoining the land of the Northern Continent the Old Red Sandstones were laid down, and the exact conditions under which they were accumulated is a matter of some interest. The late Sir Andrew Ramsay gave reasons for supposing that many red deposits were accumulated in the waters of inland lakes, which underwent rapid evaporation, and his views have been applied, with much corroborative evidence by Sir A. Geikie, to account for the red sandstones of Devonian age, which he believes to have been accumulated in a series of inland lakes, though others hold a different opinion, and consider that the Old Red Sandstone waters had a direct connexion with those of the open ocean; the question is too intricate to be discussed at length here. Besides the difference of physical characters of the two types of strata, the difference in the nature of their included organisms is significant. The ordinary invertebrates, as corals, crinoids, brachiopods and molluscs are extremely rare in the Old Red Sandstone, which contains remarkable remains of Agnatha fishes and eurypterids, and although these are also found associated with a true marine fauna in Russia, Germany and Bohemia, the rarity or apparent absence of the ordinary marine invertebrates, though only negative evidence, which is proverbially dangerous, must be regarded.

The North Devon rocks are sediments which might well be accumulated on the shores of a continent, while those of South Devon, with their abundant coral reefs, and other organic limestones were no doubt deposited in a clearer sea, at a greater distance from the land, and the clear water deposits of Germany and still more of Bohemia, were accumulated in the open ocean. It is interesting to note in these Bohemian deposits abundance of shells of a Pteropod Styliola which has been proved by Prof. H. A. Nicholson to form masses of limestone in the Devonian system of Canada. The modern distribution of the Pteropoda suggests the open ocean character of the deposits which contain them even so far back as Devonian times, though one cannot conclude that these deposits are really analogous to the so-called Pteropod ooze of modern seas which, as a matter of fact, is largely composed of foraminiferal tests with a considerable percentage of pteropod shells.