54 
DUTTON, 
neous matter and without rotation, would be truly spherical. 
If slowly rotating it would be a spheroid of two axes. If 
rotating rapidly within a certain limit, it might be a spheroid 
of three axes. 
But if the earth he not homogeneous—if some portions 
near the surface be lighter than others—then the isostatic 
figure is no longer a sphere or spheroid of revolution, but a 
deformed figure, bulged where the matter is light and de¬ 
pressed where it is heavy. The question which I propose is 
How nearly does the earth’s figure approach to isostasy ? 
Mathematical statics alone will not enable us to answer 
this question with a sufficient degree of approximation. It 
does, indeed, enable us to fix certain limits to the departure 
from isostasy which cannot be exceeded. This very problem 
has been treated with great skill by Prof. George Darwin. 
But this problem may be approached from another direc¬ 
tion with more satisfactory results. Geology furnishes us 
with certain facts which enable us to draw a much narrower 
conclusion. There are several categories of fact to which we 
may turn. One of the most remarkable is the general fact 
that where great bodies of strata are deposited they progress¬ 
ively settle down or sink seemingly by reason of their gross 
mechanical weight, just as a railway embankment across a 
bog sinks into it. The attention of the earlier Appalachian 
geologists was called, as soon as they had acquired a fair 
knowledge of their field, to the surprising fact that the 
paleozoic strata in that wonderful belt, though tens of thou¬ 
sands of feet in thickness, were all deposited in comparatively 
shallow water. The paleozoic beds of the Appalachian re¬ 
gion have a thickness ranging from 15 000 to over 30 000 feet, 
yet they abound in proofs that when they were deposited 
their surfaces were the bottom of a shallow sea whose depth 
could not probably have exceeded a few hundred feet. No 
conclusion is left us but that sinking went on pari passu 
with the accumulation of the strata. When the geology of 
the Pacific coast was sufficiently disclosed, the same fact con¬ 
fronted us there. As 'investigation went on the same fact 
