1873.] JITDH— THE SECONDARY EOCKS OP SCOTLAND, 103 



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 pakeon- 

 tological 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, some- 

 times crowded with shells of Cyrena, Unio, and other freshwater 

 bivalves, sometimes with Paludina and other freshwater univalves, 

 and at others made up of dwarfed Ostrece and other marine shells, 

 crowded together in masses, and forming beds exactly resembling 

 the well-known " Cinder-beds " of the Purbeck. As in that for- 

 mation, 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 Cyjprides and Estherice also occur, with veritable bone-bands, made 

 up of scales and teeth of fish and bones of reptiles. Not unfre- 

 quently these clays are crowded with plant-remains ; and inters testi- 

 fied with them occur beds of lignite or coal, sometimes several feet 

 in thickness, some of which have been worked with success. 



JNo 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 in- 

 discriminately referred to that age, so the peculiar characters of the 

 strata of the argillaceous type have at various times led to the an- 

 nouncement of the discovery of Wealden, Purbeck, and Phsetic 

 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 de- 

 posits from the similarity of mineral composition so strikingly illus- 

 trated as in tho 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 scries 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. 



