ROBERTSON ON THE WEALDEN BEDS OF BRORA. 127 



of similar forces during the accumulation of the Wealden series of 

 the south-east of England : — " The principal lines of elevation of 

 the Wealden are clearly referable to those movements which up- 

 heaved the chalk, and incumbent strata ; but we may observe, that 

 the deeper beds exhibit traces of extensive faults and dislocations, 

 which seem to belong to previous disruptions, for the fissures and 

 chasms are filled up with broken shale, and clay, and sand, the 

 debris of the Wealden, and contain no intermixture whatever of the 

 marine deposits which may be supposed to have once covered 

 them*." 



The "change of animal life," which a comparison of the fos- 

 sils of the "older" with those of the "younger" members of the 

 Aralo-Caspian series, and of the latter with the inhabitants of the 

 present sea, proves to have occurred, is regarded as " having accom- 

 panied the diminution of the Caspian to its present dimensions, and 

 the union of the Black and Mediterranean seas" (pp. 322-323). 

 In like manner, the alterations in the distribution and amount of the 

 waters of the Wealden basin, consequent on the elevation of its older 

 deposits, were probably the chief means of bringing about those 

 mutations of species which are observed to have taken place during 

 the accumulation of Purbeck beds, Hastings sand and Weald clay. 



The foregoing analogies between the products of the actual 

 Caspian and the hypothetical Wealden seas are, I think, in a great 

 measure confirmatory of the truth of the theory which it has been 

 my object to elucidate. No notice has hitherto been taken of the 

 Wealden series of Hanover in the present memoir. As previously 

 suggested, some of the brackish deposits of that country are probably 

 inferior to the Portland stone ; but there can be no doubt that the 

 circumstances which led to the formation of the strata incumbent 

 on the rock last named, were identical with those to which the cor- 

 responding beds of England owe their origin. The general specific 

 distinctness of the German fossils seems to indicate, that their parent 

 strata were formed in a different basin from that in which the con- 

 temporary brackish deposits of the south-east of England and north 

 of France accumulated. 



The use of the word 'Jurassic' in this memoir may be objected 

 to ; but ' oolitic ' is so inapplicable to the strata of the region to 

 which the first few pages of the preceding paper refer, that I pre- 

 ferred adopting the former term, and, having once done so, continued 

 it throughout. Every one must allow that ' oolitic,' while it is de- 

 scriptive of numerous beds in other parts of the geological scale, but 

 ill expresses the character of the majority of rocks belonging to the 

 system which it is intended to distinguish. In accordance with the 

 recent changes in geological nomenclature, a geographical term must 

 soon be applied to the system ; and while, so far as the history of the 

 science is concerned, there can scarcely be a difference of opinion as 

 to the propriety of selecting one having reference to the scene of 

 Smith's early labours, it must be remembered that Jurassic (which, 



* Dr. Mantell's ' Geology of S.E. of England,' p. 342. 



