HYPOGEIC WOEK. 379 



mountains may have great cavities beneath them, through a parting and open- 

 ing in the crustal terranes underneath, when they were elevated ; and it is 

 stated in corroboration that by means of the plumb-line it is proved that 

 the Himalayas have not the density of a solid mass. So also some volcanic 

 peaks have been proved by pendulum experiments to be hollow. If volcanic 

 mountains generally were shells over a cavity that was emptied in making 

 them, the fact that they could stand on a thin crust would be no marvel. 

 But the pendulum experiments of E. D. Preston at the Hawaiian islands 

 have shown that this is not so. He found, in 1892, that Haleakala, on east 

 Maui, 10,000 feet high, has a density of 2-7, or that of the mass of rocks at 

 the surface ; and that Mount Kea, on Hawaii, nearly 14,000 feet high, while 

 hollow above, — the density there being only 24, — has a density below of 

 3-7 (page 290). Yet these mountains stand, and, no doubt, under adjusted 

 gravitational pressure ; but how so, if on a thin crust, is an unsolved 

 mystery. 



Isostasy is earth-shaping in its action, without being mountain-making. 

 It has been in all time conservative of existing conditions of equilibrium. 

 Subsidences made by loads have cavised elevations somewhere around the 

 subsided region; but the mean level, according to the principle, must have 

 been retained. Loads over the bed of a Mexican Gulf should cause, in 

 accordance with it, a subsiding, but not a deepening, for the subsidence just 

 equals the load ; and on the border of the ocean they should cause a subsid- 

 ing of the coast region, and not a sinking; for the subsiding could not 

 exceed the filling contributed. 



The ice of the Glacial period, which covered a large part of northern 

 North America and Europe to a depth of one or more thousand feet, was a 

 load laid over the surface by moist aerial currents ; and to this load has been 

 attributed by Jamieson (1865), Warren Upham, and others, the succeeding 

 subsidence of the same glaciated regions, or that of the Champlain period. 

 (See further, page 1020). 



3. Continental plateaus and oceanic depressions. — According to the prin- 

 ciple of gravitational equilibrium, the earth's greater unevenness of surface, 

 exhibited in the existence of oceanic depressions and continental plateaus, 

 should be an expression of some difference in the density of the rocks. 

 Perhaps the fact that the prevailing rocks of the oceanic volcanoes are 

 basaltic, and of the continental, andesytic and trachytic, explains how it is 

 that the oceanic crust is made the denser. The difference in the mean den- 

 sities of the basaltic and andesytic rocks is about one tenth. The depres- 

 sions, on this view, were made in the earth's cooling. 



This origin of the oceanic basins was suggested in 1860 by Archdeacon J. H. Pratt, in 

 his memoir on the Figure of the Earth, where he attributes the existence of continents 

 and these basins to unequal contraction, refers the formation of mountains to lateral 

 pressure, and concludes that " the crust beneath the oceans is of greater density tlian the 

 average portions of the surface" ; that is, that where the contraction was greatest the 

 density of the rock material below is greatest, and proportionally so. 



