150 SOME APPLICATIONS OF 
great thickness; but a thin crust is, it seems to me, an essential condi- 
tion” (p. 543). Prof. Prestwich adduces in support of his views various 
arguments from geological phenomena which seem of much weight. He 
has also various arguments of a more or less physical character, but 
they seem to take a good deal for granted. Thus, on p. 540, refer- 
ring to plications in the surface rocks, he says, “if the earth were solid 
throughout, the tangential pressure would result not in distorting or 
crumpling, but in crushing and breaking. As a rule, no such results 
are to be seen, and the strata have - - - yielded, as only a free surface 
plate could, to the deformation caused by lateral pressure - - - a yield- 
ing bed, on which the crust could move as a separate body, was neces- 
sary.” It seems to me that as the phenomena of rupture are as yet 
very imperfectly ascertained, except perhaps for a few simple standard 
conditions, Prof. Prestwich has very little to go on but @ priori ideas. 
I fail to see, for instance, why pressures at or near the surface of a solid 
sphere should necessarily produce fracture and not flow. Also it seems 
improbable that there would be a sharp line of demarcation so as to 
enable a crust—which seems clearly to mean a solid superficial layer— 
to move as a separate body on a “yielding bed.” Would not this im- 
ply a liquid substratum with no appreciable viscosity? And supposing 
there were a substratum of this kind, is there any sufficient experi- 
mental evidence that a solid crust of even a few miles thickness would, 
on the falling away of the liquid underneath, go into folds instead of 
being crushed and broken? Further, can plications to the extent 
shown, say by the Alps, be reconciled with the retention of contempo- 
raneous solidity? Supposing the earth to be essentially solid through- 
out, is there any reason why the strain at some miles below the surface 
should not locally at intervals exceed the elastic limit, with the result 
for a time of a state of flow or plasticity throughout a volume of greater 
or less extent? During such an epoch there would exist locally condi- 
tions somewhat resembling those which Prof. Prestwich believes exist- 
ent everywhere. It is true that one argument adduced by Prof. Prest- 
wich and others against the existence of separate reservoirs of molten 
material, viz, the similarity in the character of voleanie products all 
over the earth, applies equally against such an hypothesis. If how- 
ever volcanic products be supposed to come from several miles below 
the surface, I see no obvious reason why they should not present sim- 
ilar characteristics everywhere. No conclusive argument can well be 
based on the differences observed in the sedimentary strata, because 
the conditions under which such strata are deposited are obviously of 
a varied character. 
In various passages of Prof. Prestwich’s discussion of the state of the 
earth one is apt to be puzzled by his falling into the practice, by no 
means uncommon in geological writings, of employing physical terms 
with a view to oratory rather than to exposition. For instance, he 
speaks of contraction ‘‘due to the yielding of the weaker lines in the 
