52 
MR. HOPKINS’S RESEARCHES IN PHYSICAL GEOLOGY. 
hypothesis of these investigations is thus found to be in perfect harmony with the 
results of the remoter researches contained in this and my two preceding memoirs. 
9. A question here arises as to the origin and continued existence of these insulated 
fluid masses enveloped in the solid portions of the earth. It would seem probable, I 
think, that their origin may be ascribed to the greater fusibility of the matter com- 
posing them ; and their continuance in a state of fluidity may, I conceive, be ac- 
counted for partly by the same cause, and partly by another which I will proceed to 
explain. 
Let us conceive an internal lake to have been formed as above supposed, or in any 
other manner, at a temperature which would just admit of the containing rock be- 
coming' solid, while it sufficed to preserve the fluid mass in a state of fusion. Let us 
then suppose an elevatory force, produced by the expansion of the fluid matter *, to 
raise the superincumbent solid mass, and to form in it a system of fissures. The 
plane of these fissures will scarcely ever be exactly parallel, and therefore will meet 
if sufficiently produced. Let the annexed diagram represent a transverse section of 
(!•) 
the system the instant after their formation, and before any relative displacement by 
further elevation, of the portions of the general mass contained between contiguous 
fissures, and forming so many complete or truncated wedges. The formation of these 
fissures will be completed at nearly the same instant of time-f. Conceive the mass 
to be then still further uplifted. If every portion were raised equally, the width of 
the fissures would be increased, but such will not be the case. For the complete 
wedges, not reaching down to the fluid mass, will not be immediately acted upon by 
it at all, and the truncated wedges, whose narrower sides are downwards, will be acted 
on by the fluid pressure with less force in proportion to their masses, than those of 
which the broader sides are downwards. These latter portions, therefore, will be 
more elevated than the others, and the whole will assume a position like that repre- 
sented in the following diagram. When the wedges have assumed these positions, it 
(2.) 
* Whatever difficulty there may be in fully explaining the causes of such expansion, there can be no doubt 
of the existence of such causes in aggregations of matter in a state of fusion as here supposed. The intensity 
with which they may act is attested by actual volcanos, as well as by the masses which must have been 
ejected at former epochs, in a state of fusion. 
f See memoir on Physical Geology, above referred to. 
