W. Upham — Recent Fossils near Boston. 207 



they are no longer able to live excepting in isolated colonies 

 that are preserved here and there in sheltered shallow bays. 



Looking for causes of these changes of temperature in the 

 North Atlantic and the adjoining countries, it seems to me 

 very probable that they were due mainly to a formerly larger 

 volume of the warm oceanic current which is named the Gulf 

 Stream because a considerable part of it issues from the Gulf 

 of Mexico, flowing through the Strait of Florida, while per- 

 haps a larger part leaves the tropics east of Cuba and the 

 Bahamas. This very broad current pours northward to the 

 Arctic regions and there enters an otherwise almost completely 

 enclosed ocean, from which counter currents nearly at the 

 temperature of melting ice flow back along the Labrador coast 

 and in the depths of the Atlantic under its warmer surface. 

 But within the Recent epoch, during which these climatal 

 changes have taken place, an elevation of a large region of 

 Alaska and eastern Siberia has been in progress, slowly dimin- 

 ishing the depth and width of Bering Strait.* The recency 

 of this uplifting, probably still going on, is shown, like that of 

 the basin of Hudson Bay, by drift-wood on the sea shores, 

 lying far above the level now reached by storm waves at the 

 highest tides. Mr. Dall reports that the current of the shallow 

 Bering Strait, which has a maximum depth of only 180 feet 

 and is about 36 miles wide, passes north into the Arctic Ocean ;f 

 but it may have been reversed when the strait was formerly 

 much larger, being thus an outlet for a part of the waters car- 

 ried north by the Gulf Stream. The North Atlantic and the 

 Arctic Ocean could then have received more of its northward 

 warm current, giving a milder climate to northeastern North 

 America and northwestern Europe and adjacent Arctic lands. 

 On the other hand, an outflow from the polar sea through 

 Bering Strait would be a frigid current, carrying greater cold 

 to Alaska, British Columbia, and the 'Pacific coast of the 

 United States. 



So nicely balanced are the conditions on which variations of 

 climate depend, that the former depression of Bering Strait, 

 through resulting changes in the oceanic circulation, may have 

 been a very important element, re-enforced probably by con- 

 temporaneous greater elevation of the Cordilleran mountain 

 belt from the St. Elias range to the Sierra Nevada, in causing 

 these mountains and the adjoining lower ground to bear very 

 lately, as Russell and Becker have shown, extensive glaciers or 

 even ice-sheets, which have now disappeared from the south- 



* William H. Dall, Alaska and its Resources, 1870, pp. 462-466. 

 f Ibid., p. 285. 



