Geology and Natural History. 231 



amount of contraction in cooling, solidification, and crystalliza- 

 tion is so great that it can hardly be doubted that its effective- 

 ness would be considerably increased. As a result of the unequal 

 contraction the continental ship would be no longer in isostatic 

 equilibrium. Differential vertical movement would be set up, 

 and material below the sea areas would tend to flow toward the 

 land areas, and thus tend to elevate the continents, but the aver- 

 age of the movements would be downward. Whether the con- 

 tinents absolutely or relatively rise under these stresses would 

 depend upon the average amount of contraction of the earth. It 

 is probable that this contraction would more than counteract the 

 tendency to uplift, and therefore that relative elevation, and not 

 absolute elevation would occur. The process above outlined 

 might continue until approximate isostatic equilibrium has again 

 been reached. We thus have a real cause for a very long continued 

 growth. When the hypogene forces causing more rapid subsi- 

 dence of the sea beds than of the continents, and consequently 

 relative upheaval of the continental masses, shall have finally ex- 

 hausted themselves, it cannot be doubted that the epigene forces 

 will win, and that the continental masses will be reduced to an 

 even platform slightly below the level of the sea. 



.... Summarizing, we have seen that differential subsidence 

 slowly but surely causes the continental masses to rise or fall 

 with reference to the surface of the sea. As a result of the sub- 

 sidence of great areas, smaller areas, such as the plateaus and 

 mountain ranges and systems, may be elevated. The horizontal 

 stresses, thickening the strata along the zones of plication and 

 producing the mountain systems and plateaus, are but incident to 

 the larger movements of subsidence. By vulcanism vast masses 

 of magma are introduced into the outer part of the crust of the 

 earth, or spread over its surface. The continental areas, wher- 

 ever they are above the sea, are being degraded by the wasting 

 forces of water, ice, and wind. Concurrent with these move- 

 ments is deep-seated flowage. As a result of these movements it 

 is possible that the continents may grow. The remote cause to 

 which continent-making, differential subsidence, mountain-making 

 and attendant phenomena, epigene transfer, vulcanism, and deep- 

 seated compensatory flow, are due, is the force of gravity persist- 

 ently working upon a plastic contracting mass : and therefore the 

 center of gravity of the various masses moved, is nearer the 

 center of the earth as a result of the movements." h. s. w. 



2. The Geology of Massanutten Mountain, in Virginia ; by 

 Arthur Coe Spencer, pp. 1-54, 1 map and 3 diagrams, 189G. (A 

 thesis for degree of doctor of philosophy, Johns Hopkins Univer- 

 sity.) — The selection of an isolated synclinal area such as the Massa- 

 nutten Mountain for careful study has resulted, as might be ex- 

 pected, in the testing of a large number of speculations regarding 

 mountain-building and the formation of the original series of sedi- 

 ments. The present study was suggested and guided by Mr. Bailey 

 Willis, whose extended studies of the Appalachians well fit him to 



