W. Upham — Quaternary Changes of Levels. 495 



Greenland is divided from the contiguous North American 

 continent and archipelago by a great valley of erosion, which is 

 estimated from soundings and tidal records to have a mean depth 

 of 2510 feet below sea-level for 680 miles through Davis Strait; 

 2095 feet for 770 miles next northward through Baffin Bay; and 

 1663 feet for the next 55 miles north through Smith Strait. 1 



On the Pacific coast of the United States, Professor Le Conte has 

 shown that the islands south of Santa Barbara and Los Angeles, 

 now separated from the mainland and from each other by channels 

 20 to 30 miles wide and 600 to 1000 feet deep, were still a part of 

 the mainland during the late Pliocene and early Quaternary periods. 2 



In northern California, Professor Davidson of the US. Coast 

 Survey, as cited by Spencer, reports three submarine valleys about 

 25, 12. and 6 miles south of Cape Mendocino, sinking respectively 

 to 2400, 3120, and 2700 feet below the sea-level where they cross 

 the 100 fathom line of the marginal plateau. If the land here 

 were to rise 1000 feet, these valleys would be fjords with sides 

 towering high above the water, but still descending beneath it to 

 profound depths. 3 



Farther to the north, Puget Sound and the series of sheltered 

 channels and sounds through which the steamboat passage is made 

 to Glacier Bay, Alaska, ai*e submerged valleys of erosion, now 

 filled by the sea, but separated from the open ocean by thousands 

 of islands, the continuation of the Coast Range of mountains. 

 From the depths of the channels and fjords Dr. G. M. Dawson 

 concludes that this area had a Preglacial elevation at least about 

 900 feet above the present sea-level, during part or the whole of 

 the Pliocene period. 4 



The general absence of Pliocene formations along both the Atlantic 

 and Pacific coasts of North America indicates that during this long 

 period all of the continent north of the Gulf of Mexico held a greater 

 altitude, which, from the evidence of these submarine valleys, is 

 known to have culminated in an elevation at least 3000 feet higher 

 than that of the present time. Such plateau-like uplift of the 

 whole continent appears to have exerted so great influence on its 

 meteorologic conditions, bringing a cooler climate throughout the 

 year, that it finally became enveloped by an ice-sheet to the southern 

 limit of the glacial stria?, till, and moraines, stretching from 

 Nantucket and Cape Cod to New York, Cincinnati, Saint Louis, 

 Bismarck, and thence westward to the Pacific somewhat south of 

 Vancouver Island and Puget Sound. The thickness of the ice-sheet 

 in the region of the White Mountains and the Adirondacks was 

 about one mile ; and Dana has shown, from the directions of 

 striation and transportation of the drift, that its central portion over 

 the Laurentian highlands between Montreal and Hudson Bay had 

 probably a thickness of fully two miles. In British Columbia, 



1 Smithsonian Contributions to Knowledge, vol. xv. pp. 163, 164. 



2 Bulletin of the California Academy of Sciences, vol. ii. 1887, pp. 515-520. 



3 Id. vol. ii pp. 265-8. 



4 Canadian Naturalist, new series, vol. viii. pp. 241-248, April, 1877. 



