i. 9 GEOLOGICAL CHANGES 13 



extremes of climate develop, some parts being cold, others forming 

 large, dry interior plains. 



Besides changes in the balance produced by denudation and the 

 advance of ice-caps there are also from time to time marked move- 

 ments of uplifting or lowering of the land, which may be called 

 independent earth movements. Such vertical movements of the con- 

 tinental masses are produced by internal forces of unknown origin. 

 They are doubtless related to a second series of major movements of 

 crustal deformation that are due to tangential forces and lead to the 

 formation of new mountain ranges (orogenesis) by compression, or to 

 fracturing by tension. The upwelling of lava from the inside of the 

 earth at these times makes the igneous rocks, usually devoid of fossils. 



Changes in geography are, then, mainly changes in the height of 

 the land and the amount of it that is above water. Where the con- 

 tinent is surrounded by a rather shallow continental shelf, this leads 

 to considerable changes in appearance of the land-masses. The general 

 opinion is that the main outline of the continental masses has remained 

 much as at present, at least since Cambrian times. However there have 

 probably been considerable movements of the land-masses in relation 

 to each other. Some hold that the continents of lighter material are 

 continually expanding, at least in certain directions, having grown 

 from small centres to their present size. According to the hypothesis 

 of Wegener, the continents have all been formed by the splitting up 

 of one or a few land-masses. There is indeed evidence from both 

 geophysics and biology that the continents have been drifting apart 

 (Bullard, 1959). The direction of magnetization of rocks, which is 

 determined at the time of their formation, shows that the land-masses 

 must have changed their positions greatly. For example, such data 

 show that during the Triassic period the British Isles lay in the tropics 

 and in confirmation of this we find that many salt deposits (formed 

 only in very warm climates) lie in the Triassic formation (Droitwich, 

 Bath, Nantwich, &c). 



9.2. Changes of climate 



Evidence of marked changes of climate is the finding in England 

 and other regions now temperate of animal and plant remains appro- 

 priate to warmer or colder conditions (corals and woolly rhinoceros, 

 for instance). There is thus every reason to think that there have been 

 great changes from hot to cold and wet to dry conditions, in conjunc- 

 tion with the changes in latitude and in level of the land. 



These fluctuations in geography and climate are obviously of great 



