180 



G. K. GILBERT — CONTINENTAL PROBLEMS. 



sphere. This occasion, too, in which, after three years' sojourn in the 

 land of the raccoon and 'the opossum, we return to the land of the sable 

 and the beaver, brings forcibly to mind the continental extent of our 

 society and its continental field. It is not strange, then, that the conti- 

 nents have seemed to me a fitting theme of which to speak to you to-day. 

 Realizing not only the breadth and grandeur, but the inherent difficulty 

 of the subject, I do not hope to enlarge the contribution the decade has 

 made, nor shall I attempt to summarize it ; neither is it my desire to 

 anticipate the discussions of the World's Fair congress. It is my pur- 

 pose, rather, to state, as clearly as I may, some of the great unsolved 

 problems which the continents propound to the coming intercontinental 

 congress of geologists. 



Differentiation of continental "n'l oceanic Plateaus. — It is one of the para- 

 doxes of the subject that our ideas as to the essential character of the 



♦ 30,000 FT.r- 



♦ 20.000 FT. 



♦ I0.000 FT. 



SEA LEVEL 



I 0,000 FT. 



2 0,000 FT. 



■30,000 FT. 



CONTI NENTAL 

 PLATEAU 



OCEANIC 



PLATEAU 



Figure \.— Generalized Profile, showing relative Areas of the Earth's Surface at different Heights and 



. Depths. 



continents have been greatly modified and clarified by the recent explo- 

 ration of the sea. The work, especially, of the " Challenger " and the 

 " Blake " in delineating and sampling the bottom of the ocean has given 

 new definitions, not only to the term " deep sea," but also to the term 

 " continent," as they are employed by students of terrestrial mechanics 

 and of physical geography. To the continental lands are now added the 

 continental shoals, and the depth of the deep sea is no longer its sole 

 characteristic. Look for a moment at this generalized profile of the 

 earth's surface. It expresses in a concise way the relations of area to 

 altitude, and of both to the level of the sea. Murray, to whose generaliza- 

 tions from the "Challenger" dredgings and soundings the student of 

 continents owes so much, has computed, with the aid of the great body 

 of modern data, the areas of land and ocean bed contained between cer- 



