238 THE OCEAN 



of land animals, and their slow and interrupted 

 spread along the three great south-reaching 

 continental tongues of land, than by great 

 hypothetical land-bridges. It is in like manner 

 difficult to account for the evidences of coral 

 reefs in the polar areas and of glacial periods 

 towards the equator, but it seems easier, from 

 a physical point of view, to assume a shifting 

 of the poles, a second rotation of the earth, 

 or even a change in the position of the 

 continental blocks relatively to each other as 

 well as in their geographical position on the 

 surface of the globe (possibly after the separa- 

 tion of the moon), than to accept the theory 

 that whole continents have completely dis- 

 appeared below the bed of the existing oceans. 

 The existing superficial layers of the 

 lithospherC; both on the continents and 

 beneath the oceans, appear to be parcelled 

 out into great earth-blocks, separated from 

 each other by faults and fissure-lines, along 

 which volcanic action and gaseous emana- 

 tions take place, and through which massive 

 outflows of molten matter occur, but there 

 seems little evidence to show that magmas 

 have other than a quite local extent. On the 

 whole these volcanic materials appear to be 

 lighter and more acid in composition over the 

 continental areas, and heavier and more basic 

 over the sub-oceanic areas. The continental 



