THE FOUNDATIONS OF THE CONTINENTS 83 



fore, nothing to prevent us from supposing that 

 these rocks are really remains of the lower portions 

 of the original crust which first formed on the sur- 

 face of our cooling planet, though the details of 

 their consolidation and the possible interactions of 

 heat and heated water may admit of much discus- 

 sion and difference of opinion. 



But after the formation of a crust and its cover- 

 ing in whole or in part with heated water, other 

 changes must occur, in order to fit the earth for 

 the abode of life. These proceeded from the 

 tensions set up by the contraction and expansion 

 of the interior heated nucleus and the solid crust — 

 a complicated and difficult question, when we con- 

 sider its laws and their mode of operation, but 

 which resulted in the folding and fracturing of the 

 crust along long lines which are parts of great 

 circles of the earth, running in N.E. and S.W. 

 and N.W. and S.E. directions ; and these ridges, 

 which in the earliest Archtuan period must have 

 attained to great height and very rugged outlines, 

 formed the first rudiments of our mountain chains 

 and continents. Those constituting the Laurentian 

 nucleus of North America — a very simply outlined 

 continent — form a case in point (Fig. 18). 



