strata have been hitherto regarded as contemporaneous with the Lower Oolites of Yorkshire. I shall, however, be able to show, from the manner in which the various series of estuarine beds, both on the east and west coasts of Scotland, alternate with marine strata, of which we are able to fix the age by the most conclusive palaeontological evidence, that the former belong to various periods, from the Lower Lias up to the Upper Oolite.
The argillaceous type of estuarine strata, though usually forming series of much less thickness than those of the arenaceous type, presents many features of great interest. It is characterized by finely laminated clays, usually of green, blue, grey, and black colours, sometimes more or less sandy, and passing into fire-clay, and containing impure argillaceous ironstone in bands and nodules. These laminated clays contain also thin bands of limestone, sometimes crowded with shells of Cyrena, Unio, and other freshwater bivalves, sometimes with Paludina and other freshwater univalves, and at others made up of dwarfed Ostreoe and other marine shells, crowded together in masses, and forming beds exactly resembling the well-known " Cinder-beds " of the Purbeck. As in that formation, too, we frequently find thin seams of fibrous carbonate of lime, so well known to the workmen under the name of " beef-" and "bacon-beds." In these clays beds crowded with the valves of Cyprides and Estherioe also occur, with veritable bone-bands, made up of scales and teeth of fish and bones of reptiles. Not unfrequently these clays are crowded with plant-remains ; and interstratified with them occur beds of lignite or coal, sometimes several feet in thickness, some of which have been worked with success.
No one can examine these strata of the argillaceous type without being at once struck with their resemblance to those of the Purbeck formation, and also to those of similar character which occur at the top of the Wealden in the Isle of Wight and elsewhere, which I have described in detail under the name of the Punfield Formation. As the general similarity in character of the strata of the arenaceous type to the Lower Oolites of Yorkshire has led to their being indiscriminately referred to that age, so the peculiar characters of the strata of the argillaceous type have at various times led to the announcement of the discovery of Wealden, Purbeck, and Rhaetic strata in Scotland. These strata, however, will be shown to belong to various portions of the Jurassic period ; beds of precisely similar character occur in the Lower Oolites of the Midland district of England.
Nowhere is the fallacy of inferring the contemporaneity of deposits from the similarity of mineral composition so strikingly illustrated as in the Jurassic strata of Scotland. While from such resemblances in general characters, when due allowance has been made for metamorphism subsequent to deposition, we may usually safely conclude the conditions under which the two series were respectively formed to have been similar, yet to base any argument on them as to age can scarcely fail, as in the present instance, to lead to the most serious errors.
