288 O. Fisher— Physics of the Earth's Crust. 
the features to be explained are not such as would have 
been produced by contraction. The strains set up in the 
crust by a shrinking nucleus would be such, that for any 
u 
plications in long narrow belts, with the axes of the folds 
all approximately parallel, with no corresponding plications at 
right angles to them, is an impossible result of a collapsing 
spherical shell. It certainly seems as if those who advocate 
contraction had inferred that the tangential strains set up by 
such a cause, would act only in two opposite directions ; where- 
as, since they must be uniformly distributed over the entire 
spherical surface, they must act in every direction within a tan- 
gent plane at any point. 
Mr. Fisher’s postulate of a solid crust, resting upon a plastic 
substratum, is one which seems indispensable to any rational 
theory of terrestrial physics. It will hardly be questioned by 
any geologist. Indeed, is not the proof of it abundant and 
complete? Surely no one can question the fact that the vast 
bodies of strata deposited in all areas of maximum sedimen- 
tation have sunk bodily as rapidly as they accumulated. The 
Paleozoic strata of Western Europe and Eastern America, the 
Carboniferous and Mesozoic system of the west, were accumu- 
lated in comparatively shallow waters with the surface of 
deposition almost constantly near sea-level. But if they pro- 
gressively sank in this way they must have displaced yielding 
matter beneath. How could it have been otherwise? In the 
face of a conclusion sustained by evidence so irrefragable, it is 
certainly to be hoped that no geologist will have his faith at 
all shaken by any purely theoretical conclusions which may 
have been reached by physicists in their discussions of the 
effects of tidal strains upon the earth. Mr. Fisher, however, 
proposes a very fair compromise to the physicists) He might 
be understood as saying to them, “ give me a rigid crust resting 
upon a plastic substratum, and you may do what you like wit 
the remainder.” If this concession does not meet the require- 
ments of the physicist so much the worse for the tidal argu- 
ment. In truth the position of the geologist here is incompar- 
ably the strongér of the two. The plasticity of at least a thin 
shell next below the solid external rocks has a validity of the 
highest order. Reasoning or induction scarcely enter into it— 
I 
