CONTINENTAL PROBLEMS OE GEOLOGY, 



165 



Tims ill the broadest possible way, ;iiul in a iiiaiiiicr iiraclically iiule- 

 pendeiit of the distribution of land and water, we have the ocean floor 

 clearly differentiated from the continental plateau. It is at once evident 

 that for the discussion of the i>Teater terrestrial i)robleiiis connected 

 with the configuration of the surface, and especially of the problems of 

 terrestrial mechanics, we must substitute for the continents, as limited 

 by coasts, the continental plateau, as limited by the margins of the 

 continental shoals. 



It does not follow from the profile, which, as 1 have said, represents 

 only the relation of extent to altitude, that all districts of continental 

 plateau are united in a single body, and in point of fact they are not com- 

 jjletely united; but the greater bodies are brought together, and the 

 only outlying district is that of the Antarctic continent. Running- a line 

 along- the edge of the continental shelf where a gentle slope is exchanged 

 for a stec]) one, and passing- freely, as occasion may rcfpiire, from the 

 coast down to the line of 1,000 fathoms, a continental outline is pro- 



FlorRE 2.— The contiiunUil iihitcav rt.v related to the Weitern ami h'aat, rn Tfemigphereg. 



duced in which Xorth America and Eurasia are united through the 

 shoals of the Arctic ocean, and in Miiich Australia and the greater 

 islands of the l^ast Indies are. joined to southwestern Asia. Antarctica 

 alone stands separate, being iiarted from South America by a broad 

 ocean channel, im])er(ectl\- surveyed asyet, but believed to have a depth 

 of between 1,000 and -!,()0() fathoms. The lower plateau, or the floor of 

 the deep ocean, is less continuous, being- separated by tracts of moder- 

 ate depth into three great bodies, coinciding ap])roximately with the 

 Pacific, Atlantic, and Indian oceans. 



Rifjiditi/ versus isosfasij. — The first of our continental problems refers 

 to the conditions under Avhich the differentiation of the eaitlTs surfa(;e 

 into oceanic and continental ])lateaus is ])ossible. How are the conti- 

 nents su])])orted ? l^h^ery i)ait of the oceanic plateau sustains the Aveig-ht 

 of the superjacent column of water. At the same level beneath the 

 continental idatean each unit of the lithosi)here sustains a column of 

 rock both taller and denser than the column of water and weighing 



