- v - Xature and Bearings of Isostasy. 



details are from the standpoint of the working geologist, 

 their study is comparable to the close study of the tech- 

 nique of a painting whose motive and artistic effects are 

 visible only from a distance. It is the larger conception 

 which determines the expression of the details. 



To obtain a comprehensive knowledge of the earth as 

 a basis for research, to perceive the larger principles 

 whose workings are expressed in the smaller features, 

 we must seek to understand the dynamical conditions 

 within the earth as comprehensively as we have come 

 to understand the gradational forces which work upon 

 its surface. The earth in its major divisions of con- 

 tinental platforms and ocean basins, in its subdivisions 

 of plateaus, deeps, and the folded mountain arcs, pos- 

 sesses an individuality which contrasts it to those two 

 celestial bodies, the moon and Mars, which are free from 

 concealing envelopes of cloud and near enough to show 

 us through the telescope the character of their surface 

 features. It is the relation of variable density in the 

 crust to this distinctive relief of the earth's surface 

 which is the subject matter of isostasy. 



It is a field which, as may be seen by a preliminary 

 survey, is related to many of the larger and still unsettled 

 problems of geology. Going back in thought to the origin 

 of the continents, we may note the great contrasts in 

 the hypotheses which have been put forth. Dana re- 

 garded the major relief of continent and ocean floor as 

 obtained in the original freezing of a crust and conse- 

 quent upon the faster cooling and shrinkage of certain 

 portions. Fisher advanced the supposition that in pri- 

 mordial times the earth rotated so fast that a part of the 

 siliceous crust, that of the Pacific hemisphere, broke 

 away to form the moon. Australia and the Americas 

 cracked free and floating away from the remaining por- 

 tion of the crust drifted toward the great Pacific depres- 

 sion. Suess has built up the hypothesis of continental 

 fragmentation, by which process he conceived the margins 

 of the continents to have faulted down into periodically 

 widening ocean basins. The process, according to him, 

 has continued through geological time and he looked 

 forward to a planet in which the continental relief might 

 disappear, the lands giving place to oceans and terminat- 

 ing the career, save for a few marine adaptations, of all 

 those animals . which breathe with lungs. Chamberlin, 

 on the contrary, has argued that the continental plat- 



