THE HUMAN OR PRESENT PERIOD. 521 



evidence is found in the fact that almost nowhere does the real edge 

 of the continent appear above the ocean. Very generally it lies 100 

 fathoms below sea-level, and a continental shelf almost universally 

 borders the continents. An area of 10,000,000 square miles, or more 

 than 15% of the true continental surface, is thus submerged. This 

 submergence took place so recently that the shelves are quite gen- 

 erally marked by trenches, valleys, and embayments referable to 

 rivers that formerly crossed them, and which have not yet been con- 

 cealed by sedimentation. These features imply that the continent 

 was recently so deformed that these shelves were out of water, and 

 that the rivers reached the true borders of the continental platforms. 

 They equally imply a general movement toward continental submer- 

 sion since, such, perhaps, as characterized many periods of past geologic 

 history. 



In passing, it is important to note that the almost universal pres- 

 ence of submerged continental borders has a very significant bearing 

 on the fundamental question whether continental movements are 

 simultaneous or reciprocal. If such movements were reciprocal, some 

 continents should now be in the protuberant phase, with their borders 

 as pronouncedly out of water as other borders are submerged. So, 

 retrospectively, some should show marked participation in the ele- 

 vation of the late Tertiary, while others should show as marked par- 

 ticipation in the reciprocal submersion. In fact, however, all con- 

 tinents show signs of recent protrusive movement, and all show, by 

 their trenched continental shelves, the early stages of a common move- 

 ment of the sea upon the land. 



The channels on the continental borders. — Wherever the conti- 

 nental shelves have been carefully explored by soundings, their sur- 

 faces show channels of river-like aspect, as already remarked. The 

 fjords and submerged valleys of the northern coasts are the most familiar 

 examples, as they have been much appealed to in support of the ele- 

 vation hypothesis of glaciation. 1 The data in more southerly lati- 

 tudes, especially on the Atlantic Coast from the Gulf of St. Lawrence 

 to the Antilles, have been developed and emphasized by J. W. Spencer; 2 



1 Among many others, Dana, Man. Geol., pp. 946-51; Upham, Bull. Geol. Soc. 

 Am., Vol. X, 1898, pp. 5-10. 



2 Among other papers, Bull. Geol. Soc. Am., Vol. VI, 1895, pp. 103-140, and Vol. 

 XW, 1903, pp. 207-226; Am. Jo r. Sci., Vol. XIX, 1905, pp. 1-15; and Am. Geol- 

 XXIV, 1904, pp. 110-111. 



