Cn. XX.] 



UPPER PURBECK. 



293 



The Chalk crops out from beneath the tertiary sands and clays of the 

 Paris basin, near Epernay, and the Gault from beneath the Chalk and 

 Upper Greensand at Clermont-en-Argonne ; and passing from this place 

 by Verdun and Etain to Metz, we find two limestone ranges, with inter- 

 vening vales of clay, precisely resembling those of southern and central 

 England, until we reach the great plain of Lias at the base of the Inferior 

 Oohte at Metz. 



It is evident, therefore, that the denuding causes have acted similarly 

 over an area several hundred miles in diameter, sweeping away the softer 

 clays more extensively than the limestones, and undermining these last so 

 as to cause them to form steep cliffs wherever the harder calcareous rock 

 was based upon a more yielding and destructible clay. 



UPPER OOLITE. 



Purheck beds (a, Tab. p. 291). — These strata, which we class as the 

 uppermost member of the Oolite, are of limited geographical extent in 

 Europe, as already stated, but th^y acquire importance, when we consider 

 the succession of three distinct sets of fossil remains which they contain. 

 Such repeated changes in organic life must have reference to the history 

 of a vast lapse of ages. The Purbeck beds are finely exposed to view in 

 Durdlestone Bay, near Swanage, Dorsetshire, and at Lul worth Cove and 

 the neighboring bays between Weymouth and Swanage. At Meup's 

 Bay, in particular, Professor E. Forbes examined minutely in 1850 the 

 organic remains of this group, displayed in a continuous, sea-cliflT section ; 

 and he added largely to the information previously supplied in the works 

 of Messrs. Webster, Fitton, De la Beche, Buckland, and Mantell. It ap- 

 pears from these researches that the Upper, Middle, and Lower Purbecks 

 are each marked by peculiar species of organic remains, these again being 

 different, so far as a comparison has yet been instituted, from the fossils of 

 the overlying Hastings Sands and Weald Clay.* 



Upper Purheck. — The highest of the three divisions is purely fresh- 

 water, the strata, about 50 feet in thickness, containing shells of the 

 genera Paludina^ Physa^ Limnceus, Planorbis, Valvata, Cyclas, and 

 Unio^ with Cyprides and fish. All the species seem peculiar, and among 

 these the Cyprides are very abundant and characteristic. (See figs. 

 334, a, Z*, c) 



Fig. 334 



Cyprides from the Upper Purbecks. 

 a. Cypris gibbom,'E.'FoThes. I. C)/pristuberculaia,'E.Fov'be3. c. Ci/prislegumineUa,KFo!:hes. 



"On the Dorsetshire Purbecks," by Prof. E. Forbes, Brit. Assoc. Edinb, 1850. 



