MR. HOPKINS’S RESEARCHES IN PHYSICAL GEOLOGY. 
51 
always found to be small compared with r x ; and by analogy we conclude that such 
must be the case also with respect to the matter composing the earth, and under the 
pressure to which it is subjected at great depths. We may also remark, that the po- 
sition of a surface of equal solidity or fluidity must necessarily so far incline to the 
corresponding surface of equal temperature as to differ materially from that of equal 
density, so that the real effective thickness of the crust is probably considerably 
greater than its inferior limit as above determined. Upon the whole, therefore, we 
may venture to assert that the minimum thickness of the crust of the globe which can 
be deemed consistent with the observed amount of precession, cannot be less than 
one-fourth or one-fifth of the earth’s radius. 
§. Constitution of the Eartlis Crust. 
8. The results at which w T e have arrived respecting the thickness of the solid crust 
of the globe, have an important bearing on our physical theories of volcanic forces, 
and the mode in which they act, whether we consider the subject with reference to 
existing volcanos, or to that more general volcanic action to which we refer all the 
geological phenomena of elevation. Many speculations respecting actual volcanos 
have rested on the hypothesis of a direct communication, by means of the volcanic 
vent, between the surface and the fluid nucleus beneath, assuming the fluidity to 
commence at a depth little, if at all, greater than that at which the temperature may 
be fairly presumed to be such as would suffice, under merely the atmospheric press- 
ure, to fuse the matter of the earth’s crust*. W T hen it is proved, however, that that 
crust must be several hundred miles in thickness, the hypothesis of this direct com- 
munication is placed, as I conceive, much too far beyond the bounds of all rational 
probability to be for an instant admitted as the basis of theoretical speculations. We 
are necessarily led, therefore, to the conclusion that the fluid matter of actual vol- 
canos exists in subterranean reservoirs of limited extent, forming subterranean lakes, 
and not a subterranean ocean. Such also we conclude from the present thickness of 
the earth’s crust, must have been the case for enormous periods of time ; and, conse- 
quently, that there is a very high degree of probability that the same was true at the 
epochs of all the great elevations which we recognize, with the exception, perhaps, of 
the earliest. If, moreover, we find that the hypothesis of the existence of these sub- 
terranean lakes at no great depth beneath the surface, does enable us to account 
distinctly, by accurate investigations founded on mechanical principles, for the pheno- 
mena of elevation and the laws which they follow, then have we all the proof of the 
truth of our hypothesis which the nature of the case will admit of. These investiga- 
tions I have given in my memoir on Physical Geology, published in the sixth volume 
of the Transactions of the Cambridge Philosophical Society. The fundamental 
* Some of the most ingenious and determinate speculations of this kind are contained in a paper by Professor 
Bischoff, in the Edinburgh New Philosophical Journal for 1838—39. His views respecting the immediate 
agency by which volcanic action is produced will be equally applicable, whether the reservoir of volcanic matter 
beneath be of limited extent, or the central nucleus itself. 
H 2 
