UPHEAVALS. 167 



in favor of Professor Dana's theory, though some geologists do not accept 

 it. If this theory were absolutely disproved, it would still be impossible 

 to suppose that upheaval and subsidence everywhere exactly balance each 

 other in the long ran. If continents once existed where the great oceans 

 now lie, a perfect history of the earth would show that there were contir 

 nents in some parts of the world through larger portions of geological time 

 than in other regions. In regions where the total erosion has exceeded the 

 total sedimentation, the original crust must almost certainly be exposed. 



Bearing of principle of hydrostatic equilibrium. Notlliug UX geology is HlOrC CCrtaiu 



than that the earth is very nearlj' in a condition of hydrostatic equilibrium,' 

 and it is the maintenance of this equilibrium which necessitates upheaval 

 and subsidence. This is perfectly evident if the interior of the earth is fluid. 

 It is also true if the earth is solid to the center and as rigid as steel or 

 glass; for a mass as large as the eartli of either of these substances could 

 not maintain a shape diverging considerably from a- form of fluid equilibri- 

 um for any length of time. Even masses of metal of a few tons (e. g., 

 metallic mirrors for astronomical purposes) undergo deformations by their 

 own weight. So also will a slab of marble supported at its extremities, 

 and, in short, the flow of solids in general is a well recognized fact.- Now, 

 if the earth is a solid, highly viscous mass, as Thomson and Darwin have 

 concluded, the effect of the subsidence of, say, a sedimented oceanic area 

 must be felt to the center of the earth, and the earth from the center to the 

 surface must partake in an upheaval. If, on the other liaiid, the globe con- 

 sists of a solid .shell, which is growing thicker, and a fluid ball upon which 

 the shell floats, the effect of the subsidence of a given area must be to 

 depress the fluid magma underlying this area and to raise some other 

 column of the fluid under eroded regions. Even in this case, then, at least 

 the superficial portion of the fluid ball partakes in the movement attending 

 upheaval and subsidence. 



' Babbage, I believe, was tbe first to point out this now familiar fact. 



^ 111 discussing the question of the solidity of the earth, geologists seem sometimes to forget that 

 time enters into the conception of viscosity. The earth may be as rigid as steel with reference to forces 

 which rapidly change their directions like those exerted by the sun and moon, but as plastic as putty 

 to much smaller stresses acting continuon.sly through long periods of time in a single direction. The 

 rigidity of the earth claimed for it by physicists is not inconsistent with the flexure of strata. So a 

 stick of sealiugwas may be slowly contorted by its own weight, hut a smart blow will break it like 

 glass. 



