324 J. LE CONTE — ATLANTIC AND PACIFIC COAST CHANGES. 



running from the mouth of St. Lawrence river through St. Lawrence bay 

 and over the submarine plateau to the deep ocean-basin ; (2) Another, 

 equally distinct, extends from the mouth of the Hudson river through New 

 York harbor and out to deep water ; (3) Still another stretches from the 

 mouth of the Delaware through Delaware bay and thence seaward ; (4) 

 Officers of the U. S. Coast Survey have recently located a well defined chan- 

 nel in Chesapeake bay and beyond its opening into the Atlantic ; (5) Be- 

 yond the mouth of the Mississippi, even beyond its submarine delta deposit, 

 a similar channel is found running out to the margin of the abyssal region 

 of the Gulf of Mexico. Doubtless others will hereafter be found. 



These are all off the mouths of existing rivers and are direct continuations 

 of the adjacent river valleys, or rather of the much greater Tertiary river- 

 channels underlying the present river channels. They are the submerged 

 lower portions of the old Tertiary river beds. 



Coast Changes. — As long ago pointed out by Dana and more recently by 

 Dr. Spencer, the submarine channels are evidence of a former more elevated 

 condition and wider extent of the continent. They were hollowed out by 

 erosion at a time when the continent stood much higher than now and the 

 shore-line was at or beyond the present 100-fathom line. The former eleva- 

 tion shown in this way is estimated by Spencer as at least 2,000 to 3,000 feet 

 above the present condition. 



The hollowing of these channels was undoubtedly the work of the Tertiary, 

 and probably of the later Tertiary. The continental elevation probably 

 commenced in the late Miocene, increased through the Pliocene, and cul- 

 minated at the end of the Pliocene, or more probably in the early part of the 

 Quaternary, and was at least one of the causes, perhaps the main cause, of 

 the glacial cold of northern regions. Following the elevation and probably 

 assisted by the weight of the accumulating ice-sheet, there was a subsidence 

 which carried the continent below its present level. From this depressed 

 condition, as the ice was removed, the continent rose again to its present 

 position, which, however, is far below that at the end of the Tertiary and 

 beginning of the Quaternary. This fact, I may remark, is proof positive 

 that there were other causes of subsidence besides mere v.'eight of ice. 



Such are, in brief, the phenomena and such the explanation, on the eastern 

 coast. On the western coast there are also found submarine channels; but in 

 those off the California coast there are some significant peculiarities. 



The Pacific Coast and its Changes. 



Physical Geography. — Commencing northward, we find off the mouth of 

 Puget sound and the Gulf of Georgia (i. e., off the Strait of Fuca) a wide 

 and deep channel cutting into the submarine plateau and passing seaward 



