O. Fisher —Physics of the Eartl’s Crust. 285 
explanation. Within the earth we know of heat and gravita- 
tion as possible sources of such energy. Have these really 
been the agents, and if so, in what specific manner have they 
acted? The compression to which strata seem to have been 
subjected, is very generally explained by the hypothesis that the 
interior of the globe has contracted through secular cooling, 
while the crust collapsing upon the shrinking nucleus becomes 
wrinkled and distorted. Mr. Fisher proceeds to compute the 
intensity of the compressive force which would be generated in 
a tangent to the crust upon this supposition, and finds it to be 
. about 830,000 tons upon the square foot. So far as intensity 
is concerned, there can be no doubt of the sufficiency of this 
pressure. 
Having shown the sufficiency of this factor he then pro- 
ceeds to inquire (Chapter V) whether the work has really been 
accomplished in this way. He ins by seeking for some 
measure of the inequalities of the surface; taking first the 
greater inequalities; the oceanic basins and continents. If the 
existing inequalities of the surface were leveled down and 
spread out they would, he estimates, form a layer over the 
whole earth from 9,500 to 13,000 feet thick—an under- rather 
than an over-estimate. 
e 2 many things will become explicable. From a 
liquid or plastic substratum it would follow that the position of 
