EVIDENCES OF UPHEAVAL AND DEPRESSION. 



55 



On the right of the names of the strata are the lithological 

 names of groups into which they are arranged. On the left of the 

 column of strata are the names of the prevailing mineral substances 

 of which they consist, and in the succeeding columns farther to the 

 left are the theoretical deposits which may be supposed to have 

 taken place in adjacent areas if they were always thrown down 

 in the order here given ; and it may be noticed that the land recedes 

 farther from the vertical section from the Trias up to the middle 

 of the Oolites, and then, by elevation, approaches again the area which 

 is now the south of Britain. The Cretaceous rocks are a group of 

 another kind, and include within themselves subdivisions showing 

 three types of mineral character. 



Throughout the period of the Psammolithic series, the same 

 rocks continued to be denuded, as is proved by some fossils they 

 contain which do not belong to the formation, but have been 

 derived from older strata. This group in Yorkshire becomes re- 

 placed horizontally by a pelolithic representative called the Speeton 

 Clay, which being marine, shows no trace of the minute subdivisions 

 which have been recognised in the South of England, and often 

 made the basis of classification. It is a continuous clay between the 

 top of the Kimmeridge Clay and the lower Cretaceous rocks. Simi- 

 larly the Pelolithic group in Yorkshire shows towards its base a 

 tendency to become psammolithic in the Kelloway rock, and 

 southward in the section, at Boulogne, its upper part is putting on 

 Psammolithic characters. The Oolitic group when traced towards 

 Yorkshire becomes in the main psammolithic, thus demonstrating 

 the direction in which land existed relatively to the British area 

 which was being covered with these deposits, and enabling us to 

 infer, with the aid of larger knowledge of European formations, the 

 essential directions of the ancient coast-lines. 



A TABLE OF THE CHIEF BRITISH STRATA 



Arranged in the order in which they rest upon each other, with an 

 indication of the prevalent mineral character of the beds, and some 

 of their chief variations. 



