1873.] JT/DD — THE SECONDARY ROCKS OP SCOTLAND. 101 



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 cha- 

 racter. In the latter area the whole series, from the hase 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, dis- 

 tance from the shore, and nature of sediment, are yet all of un- 

 doubted marine origin. It is only at the commencement of this 

 great period, in the Phsetic, 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- 

 Prench 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 cha- 

 racter and insignificant in extent. 



In Yorkshire, however, there is evidence that, during the whole 

 period of the Lower Oolite, estuarine conditions prevailed over a con- 

 siderable 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 Avith strata deposited under these conditions, the 

 succession of beds is found to undergo great changes within com- 

 paratively 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 Carboni- 

 ferous system. There, as is so well known, the marked distinction 

 between the marine strata of the Mountain-limestone at the base, 



