234 



LYELL'S ELEMENTS OF GEOLOGY. 



Valleys and Escarpments of Chalk, Oolite, and Lias. 



place ; so that, in each set of strata, from the Upper Oolite to 

 the Lias, some peculiar and characteristic fossils were imbedded. 



Physical geography. — The alternation, on so large a scale, 

 of distinct formations of clay and limestone, has given rise to 

 some marked features in the physical outline of parts of Eng- 

 land and France. Wide valleys can usually be traced through- 

 out the long bands of country where the argillaceous strata 

 crop-out ; and between these valleys the limestones are observed, 

 composing ranges of hills, or more elevated grounds. These 

 ranges terminate abruptly on the side on which the several clays 

 crop-out from beneath the calcareous strata. 



The annexed diagram will give the reader an idea of the con- 

 figuration of the surface now alluded to, such as may be seen in 

 passing from London to Cheltenham, or in other parallel lines, 

 from east to west, in the southern part of England. It has been 



Fig. 231. 



Inferior Middle Upper London 



Oolite. Oolite. Oolite. Chalk, clay. 



Lias. Oxford clay. Kim. clay. Gault. 



necessary, however, in this drawing, greatly to exaggerate the 

 inclination of the beds, and the height of the several formations, 

 as compared to their horizontal extent. It will be remarked, 

 that the lines of cliff, or escarpment, face towards the west in 

 the great calcareous eminences formed by the Chalk and the 

 Upper, Middle, and Lower Oolites ; and at the base of each we 

 have respectively the Gault, Kimmeridge clay, Oxford clay, and 

 Lias. This last forms, generally, a broad veil at the foot of the 

 escarpment of Inferior Oolite; but a considerable portion of 

 that escarpment is sometimes occupied by lias. The external 

 outline of the country which the geologist observes in travelling 

 westward from Paris to Metz, is precisely analogous, and is 

 caused by a similar succession of rocks intervening between the 

 tertiary strata and the Lias ; with this difference, however, that 

 the escarpments of Chalk, Upper, Middle, and Inferior Oolites, 

 face towards the east instead of the west. 



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 Green-sand at Clermont en Ar- 

 gonne ; and passing from this place by Verdun and Etain to 

 Metz, we find two limestone ranges, with intervening vales of 

 clay, precisely resembling those of southern and central Eng- 



