Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/145

 great completeness the history of the Jurassic system in Scotland, and to furnish many details with regard to that of the Triassic and Cretaceous.

I. General Characters of the Jurassic Strata of Scotland.

When we compare the Jurassic series as displayed in Scotland with that of the typical district of the south of England, we find many differences of a very striking and highly suggestive character. In the latter area the whole series, from the base of the Lower Lias to near the top of the Upper Oolite, is represented by strata which, while exhibiting evidence of having been deposited under very various conditions, determined by depth of water, distance from the shore, and nature of sediment, are yet all of undoubted marine origin. It is only at the commencement of this great period, in the Rhaetic, and towards its close, in the Portland and Purbeck strata, that we find evidence of the estuarine conditions which afterwards prevailed during the deposition of the Anglo- French Wealden far into the Neocomian period.

As we proceed northwards into the Midland district of England, the Jurassic system begins to exhibit several intercalated series of beds with estuarine characters ; but these, though of great interest in themselves, are nevertheless, as compared with the great mass of marine strata with which they are associated, subordinate in character and insignificant in extent.

In Yorkshire, however, there is evidence that, during the whole period of the Lower Oolite, estuarine conditions prevailed over a considerable area, and a series of strata was deposited consisting of sandstones, shales, ironstones, and thin seams of coal, which attains to a thickness of about a thousand feet ; the marine beds associated with these are of a subordinate and local character. The other Jurassic strata in Yorkshire are of purely marine origin.

In Scotland, as I shall show in the present memoir, this gradual change of character in the Jurassic system as we go northwards, is carried still further. I shall have to describe the occurrence there, from the base of the Lower Lias up to and including the Upper Oolite, of a number of series of beds exhibiting estuarine characters. These alternate with marine strata, which, however, are often of very subordinate character and limited thickness. As is usually the case with strata deposited under these conditions, the succession of beds is found to undergo great changes within comparatively short distances ; and thus the sections, at points not very remote from one another, often exhibit very remarkable contrasts. In some places the strata of estuarine origin are found to greatly exceed in thickness those of marine character, while in others the former are subordinate to the latter.

This gradual change in character of the Jurassic series, as we pass from south to north, finds a singular parallel in the Carboniferous system. There, as is so well known, the marked distinction between the marine strata of the Mountain-limestone at the base,