176 THE STORY OP THE EARTH AND MAN. 



in the earth's crust were formed. At least this is the 

 case if we reckon mere thickness. For aught that 

 we know, the Eozoic time may have accumulated as 

 much rock as the Paleeozoic; but leaving this out of 

 the question, the rocks of the Palaeozoic are vastly 

 thicker than those of the Mesozoic and Cainozoic 

 united. Thus the earth's history seems to have 

 dragged slowly in its earlier stages, or to have 

 become accelerated in its latter times. To place it 

 in another point of view, life changes were greater 

 relatively to merely physical changes in the later 

 than in the earlier times. 



The same law seems to Lave obtained within the 

 Paleeozoic time itself. Its older periods, as the 

 Cambrian and Lower Silurian, present immense 

 thicknesses of rock with little changes in life. Its 

 later periods, the Carboniferous and Permian, have 

 greater life-revolution relatively to less thickness of 

 deposits. This again was evidently related to the 

 growing complexity and variety of geographical con- 

 ditions, which went on increasing all the way up to 

 the Permian, Avhen they attained their maximum for 

 the Paleeozoic time. 



Again, each age was signalized, over the two great 

 continental plateaus, by a like series of elevations and 

 depressions. We may regard the Siluro-Cambrian, 

 the Silurian, the Devonian, the Carboniferous, and Per- 

 mian, as each of them a distinct age. Each of these 

 began with physical disturbances and coarse shallow 

 water deposits. In each this was succeeded by sub- 



