174 SEA AND LAND 



we commonl)- find that there are Init few and inadequate 

 shelters for ships ; thus on the west coast of the Americas 

 from northern Mexico to southern Chile, an essentially river- 

 less region, embayments suitable for havens are very rare. 

 Similar conditions prevail in all the seaboard regions near the 

 great deserts of north Africa. Although these shores are 

 subject to constant oscillation, they have no large troughs 

 such as the rivers cut on other coast-lines which may be filled 

 by the sea waters during the periods of subsidence. Where 

 the river valleys are deep the accumulations of debris formed 

 in them during the periods of temporary subsidence are often 

 not sufficient to obliterate their form. Thus, in the case of 

 the Hudson and other valleys along the east and west coasts 

 of North America, the depression occupied by the stream in 

 former periods of elevation is still traceable as a deep trough 

 for fifty miles or niore into the sea. If the shore near New 

 York were elevated even as much as two hundred feet, the 

 reentrant of the Hudson Valley would still appear as a deep 

 bay and afford, as it does at present, a serviceable port. Thus 

 it comes about that in periods of elevation, as well as of 

 depression, the irregularities of the coast-line, due to the 

 action of rivers, serve to maintain the varying outline of the 

 shore on wliich the existence of harbors depends. While 

 there are many other actions which operate to bring about 

 the formation of harbors, this which we are now considering 

 is probabh of more general importance and affects a larger 

 part of the coast-lines than any other class of actions at work 

 along the shores. 



In certain parts of the world in regions of high latitude, 

 we find the coast-line fretted with very numerous channels 

 which often penetrate far into the land, while the shore for 



