18 



PHYSIOGEAPHIC GEOLOGY. 



also southward to Cape Hatteras. Off the Carolinas it narrows much ; but 

 in the Gulf of Mexico it has its usual width. At times in geological history- 

 it has been part of the actual dry border of the continent. This is proved 

 by the existence of a river-channel, that of the Hudson, over its submerged 

 surface, as shown on the accompanying map of the Atlantic border. As 

 here seen, the depth of water over this border is not 50 fathoms (300 feet) 

 until within 15 miles of the 100-fathom line. 



Long Island Sound, '., 

 Long Island 



and the 



Atlantic Border 



.with :' \ 



Depths along Bathymetrle - ftr ■ 

 lines in fathoms ; Cotidal : s. 

 Unesin Long Island SoiiBil.tl—^^. j- 



the under-water Channel;; "bj] ^^ '^/ 

 of Hudson jvner, AiTjui;^.'i,v^ ^.a^ ■-VVSIlx^' 



^_i. 



10 



20 



30 



S'C-An.rOP'MTLES 

 10 50 60 



70 



80 



90 



^ 



Map of the Atlantic border. 



On the Pacific side of both jSTorth and South America the submerged 

 continental border is narrow. Off California, the distance to the 100-fathom 

 line is in general only about 10 miles. There is then a sharp descent to 

 500 or 600 fathoms, and from this a decline of 1600 to 2400 fathoms within 

 40 or 50 miles. This is in great contrast with the Atlantic border. G. 

 Davidson, of the Coast Survey, reports the existence of several deep 

 submarine channels leading outward from the coast, which are most proba- 

 bly due to streams that flowed along them at some time when the land 

 stood much above its present level. 



