182 G. K. GILBERT — CONTINENTAL PROBLEMS. 



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

 for a steep one, and passing freely, as occasion may require, from the 

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

 in which North America and Eurasia are united through the shoals of 

 the Arctic ocean, and in which Australia and the greater islands of the 

 East Indies are joined to southwestern Asia. Antarctica alone stands 

 separate, being parted from South America by a broad ocean channel, 

 imperfectly surveyed as yet, but believed to have a depth of between 

 1,000, and 2,000 fathoms. The lower plateau, or the floor of the deep 

 ocean, is less continuous, being separated by tracts of moderate depth 

 into three great bodies, coinciding approximately with the Pacific, Atlan- 

 tic and Indian oceans. 



Rigidity versus Isostasy. — The first of our continental problems refers to 

 the conditions under which the differentiation of the earth's surface into 

 oceanic and continental plateaus is possible. How are the continents 

 supported ? Every part of the oceanic plateau sustains the weight of 

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

 nental plateau each unit of the lithosphere sustains a column of rock 

 both taller and denser than the column of water, and weighing about 

 three times as much. The difference between the two pressures, or the 

 differential pressure, is about 12,000 j:>ounds to the square inch, and this 

 force, applied to the entire area of the continental plateau, urges it down- 

 ward and urges the oceanic plateau upward. Referring again to the dia- 

 gram in figure l,the entire weight of the continental plateau, pressing on 

 the tract beneath it, tends to produce a transfer of material in the direc- 

 tion from left to right, resulting in the lowering of the higher plateau 

 * and the raising of the lower. To the question, how this tendency is 

 counteracted, two general answers have been made : first, that the earth, 

 being solid, by its rigidity maintains its form ; second, that the materials 

 of which consist the continental plateau and the underlying portions of 

 the lithosphere are, on the whole, lighter than the materials underlying 

 the ocean floor, and that the difference in density is the complement of 

 the difference in volume, so that at some level horizon far below the sur- 

 face the weights of the superincumbent columns of matter are equal. 

 The first answer regards the horizontal variations of density in the earth's 

 crust as unimportant; the second regards them as important. The first 

 may be called the doctrine of terrestrial rigidity ; the second has been 

 called the doctrine of isostasy. At the present time the weight of opinion 

 and, in my judgment, the weight of evidence lie with the doctrine of 

 isostacy. The differential pressure of 12,000 pounds per square inch suf- 

 fices to crush nearly all rocks, and it may fairly be questioned whether 

 there are any rock masses which in their natural condition near the sur- 



