378 PHYSICAL GEOGRAPHY OF THE OOLITE. [Ch. XX. 



The Upper oolitic system of the above table has usually the Kimine- 

 ridge clay for its base ; the Middle oolitic system, the Oxford clay. The 

 Lower system reposes on the Lias, an argillo-calcareous formation, which 

 some include in the Lower Oolite, but which will be treated of separately 

 in the next chapter. Many of these subdivisions are distinguished by pe- 

 culiar organic remains ; and, though varying in thickness, may be traced 

 in certain directions for great distances, especially if we compare the part 

 of England to which the above-mentioned type refers with the northeast 

 of France and the Jura mountains adjoining. In that country, distant 

 above 400 geographical miles, the analogy to the accepted English type, 

 notwithstanding the thinness or occasional absence of the clays, is more 

 perfect than in Yorkshire or Normandy. 



Physical geography, — The alternation, on a grand scale, of distinct for- 

 mations of clay and limestone has caused the oolitic and liassic series to 

 give rise to some marked features in the physical outline of parts of Eng- 

 land and France. Wide valleys can usually be traced throughout the 

 long bands of country where the argillaceous strata crop out ; and be- 

 tween 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 rise up from beneath the calcareous strata. 



The annexed cut will give the reader an idea of the configuration 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 necessary, however, in this drawing, greatly 



Fig. 36T. 



Lower Middle Upper London 



Oolite. Oolite. Oolite. Chalk, clay. 



Lias. Oxford Clay. Kim. clay. Gault. 



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 which we have respectively the Gault, 

 Kimmeridge clay, Oxford clay, and Lias. This last forms, generally, a 

 broad vale at the foot of the escarpment of inferior oolite, but where it 

 acquires considerable thickness, and contains solid beds of marl-stone, it 

 occupies the lower part of the escarpment. 



The external outline of the ' country which the geologist observes in 

 travelling eastward 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 Lower Oolites face towards the east instead 

 of the west. 



