THE FOUNDATIONS OF THE CONTINENTS 85 



Then began the great processes of denudation 

 and sedimentation to which we owe the succeeding 

 rock formations. The rains descended on the 

 mountain steeps, and washed the decaying rocks 

 as sand, gravel and mud into the rivers and the 



Fig. 18. — l\Tap of Laurentian, North America. 

 Showing the protaxis or nucleus of the continent. 



sea. The sea itself raged against the coasts, and 



cut deeply into their softer parts ; and all the 

 detritus thus produced by atmospheric and marine 



denudation was spread out by the tides and 



currents in the bed of the ocean, and its gulfs and 



