230 Mr. Hopkins’s Abstract of his Memoir on Physical Geology. 
attributable to the action of some general cause, at least as ex 
tensive in its operation as the district throughout which the 
phenomena are observed to follow the same law without 
breach of continuity. This cause is assumed to be that which 
naturally suggests itself to the mind of every geologist, viz. an 
elevatory force acting simultaneously at every point of a por- 
tion of the earth’s crust, of at least the extent just intimated, 
and of any assigned thickness. It is manifest, that the eleva- 
tion of this mass must produce in it extension, and consequent 
tension, which, if of sufficient intensity, will cause those dis- 
locations or fissures which we now recognise in the phzeno- 
mena already alluded to. These fissures must, according to 
this theory, be regarded as the primary phenomena, with 
which all the other phzenomena of elevation, as faults, mineral 
veins, anticlinal lines, &c., are connected as secondary ones. 
I have carefully abstained in my memoir from any specula- 
tions on the causes which might produce this elevatory force— 
I merely assume its existence. It is easy, however, to con- 
ceive such a force to act as above supposed, if we assume the 
existence of a cavity beneath the elevated mass, either origi- 
nally coextensive with it, or rendered so by the action of the 
elevatory force itself. Any vapour or matter in a state of 
fluidity from heat, forced into this cavity, or expanded there, 
will produce the elevatory force which I assume to have acted. 
This appears to be the simplest mode in which we can con- 
ceive such a force to be produced; and if we choose to set 
out from the more remote hypothesis of the earth’s having 
been originally fluid, it might probably be shown that the 
formation of cavities such as above supposed, would, under 
simple conditions, be the necessary consequence of that pro- 
cess of cooling by which we must then suppose the crust of 
the globe to have assumed its present solidity. Instead, how- 
ever, of assuming the existence of a cavity, we might suppose 
a portion of the solid matter of the earth, at a certain depth 
beneath its surface, to become by some means expanded, and 
by its expansion to elevate the superincumbent mass. This 
hypothesis, as far as my investigations are concerned, would 
equally suffice, as, in fact, would any other by which we could 
account for the simultaneous action of an elevatory force upon 
a portion of the earth’s crust of sufficient extent. For many 
reasons, however, independent of my immediate object, I 
should not hesitate to reject this latter hypothesis as generally 
insufficient to account for observed phzenomena, and as in- 
volving serious physical difficulties. If we adopt the hypothesis 
of internal cavities, we may observe that there is no reason 
why we should not suppose them to exist, not only at differ- 
