ROBERTSON ON THE WEALDEN BEDS OF BRORA. 12 



south-east of England have not been found beyond the limits of the 

 Portland stone*. These facts are readily understood, when it is 

 considered that in the one case the Miocene beds, and in the other 

 the Portland stone, were accumulating when the elevations happened 

 which converted the bottoms of the respective oceans into those of 

 inland seas. 



Transitions between the Miocene and Aralo-Caspian strata are 

 stated (p. 304) to be "peculiar to the western boundaries of the 

 latter," i. e. as is clearly explained, to the region where the ocean 

 and Aralo-Caspian Sea communicated with each other, before the 

 elevation of the barrier which afterwards divided them. From the 

 expression "peculiar" it may be inferred that, in other parts of the 

 basin, the plane of separation between the oceanic and brackish 

 water deposits is, as in the case of the Portland stone and Purbeck 

 beds, distinguishable. 



The mineral characters of the Aralo-Caspian rocks of the Crimea 

 are thus summed up (p. 301) : — Courses of argillaceous marls, clays, 

 calcareous marls, concretions, ferruginous bands, agglutinated shells 

 (faluns), and soft spongy shelly limestone." The Wealden deposits 

 are quite as diversified, and may, as a whole, be described in nearly 

 the same words. 



The examination of the Aralo-Caspian deposits has produced 

 the conviction that they were all (p. 304?) " accumulated under one 

 vast inland sea, the inhabitants of which differed as essentially from 

 those of the ocean of that day, as the animals of New Holland now 

 differ from those of the rest of the world." The value of this passage, 

 as corroborative of my hypothesis, will be appreciated, when it is 

 recollected that the inhabitants of the Wealden sea were assumed to 

 have been quite distinct from those of the contemporary (lower 

 greensand) ocean. 



The Miocene rocks of the South of European Russia may, in 

 almost every particular, be compared to the Portland stone; the 

 Aralo-Caspian and Caspian deposits, to the Wealden beds ; and the 

 oceanic strata, now being accumulated in the Black Sea, &c., to the 

 first formed portions of the lower greensand. There is, however, 

 one remarkable difference between the two series, for the Portland 

 stone is quite as well developed beneath the Wealden deposits as 

 elsewhere, while the marine Pliocene beds of other countries are 

 unrepresented below the Aralo-Caspian strata ; in fact, these last- 

 mentioned rocks are themselves believed to be (p. 323) " the equi- 

 valents of Pliocene and Post-pliocene deposits." In the former case, 

 therefore, there seems to have been a great and sudden change in 

 the oceanic inhabitants of Europe ; while in the latter, with some 

 local exceptions, the mutation of races has been gradual and unin- 

 terrupted. 



I have already stated my belief that only one cause, viz. de- 

 siccation of the ocean bed, can account for such an extinction of 

 marine animals as happened at the close of the Jurassic period, 

 ao ^197181.0 a./ "3 J39i oj 'ifiecsqi 



3iil lo >^^*=i)i.pFiltfeiri: l^anis. Geo!. Soc, 2nd Ser., vol. iv. p. 332. 



