234 Mr. Hopkins’s Abstract of his Memoir on Physical Geology: 
ficult to calculate with any precision the resulting phenomena. 
Cases however may easily be conceived without such diffi- 
culty, though more complicated than the simple ones above, 
alluded to. Suppose, for instance, recurring to ‘our hypothesis 
of internal cavities, one cavity of great extent to exist at a cer-. 
tain depth, and another smaller one within the mass above the; 
former, and communicating with it, so that any fluid pressure 
acting in the lower should be communicated immediately to 
the upper one. ‘That portion of the elevated mass which lies 
directly above the upper and smaller cavity, may manifestly 
be subjected simultaneously to the tension impressed upon 
the whole mass from the action of the elevatory force in the 
larger cavity, and to that produced by the partial elevation 
above the smaller one. ‘These two sets of tensions may be 
conceived to be superimposed the one on the other, in the 
same manner as any two sets of forces in equilibrium may be 
so superimposed *. ‘Their intensities and directions will de- 
pend on the forms of the general and partial elevations re- 
spectively. ‘Thus we may have a partial elevation of which 
a cone or segment of a sphere should be the approximate 
type, superimposed upon a general one of which the type 
should be the segment of a cylinder. Other combinations 
might be formed in a similar manner. 
Should it appear preferable to consider the subject inde- 
pendently of the hypothesis of internal cavities, we have only, 
to conceive our partial elevations to be produced by a more 
intense action of the elevatory force at those points. As re- 
ards the resulting state of tension, it is perfectly immaterial 
which hypothesis we adopt. 
The states of tension above described refer to the mass in its 
elevated but unbroken state, z.e. previously to the formation 
of those fissures which must of course be formed when the 
tension shall become greater than the cohesive power of the 
mass. ‘The tension will begin to be produced at the instant 
the act of elevation commences, and will increase till it ac- 
quires the intensity just mentioned. Z7zme will be necessary 
for this, but it may possibly be so short as to give to the ac- 
tion of the elevatory force the character of an impulsive action, 
which would probably produce the most irregular phzeno- 
mena, and such as would be altogether without the sphere of 
calculation. I exclude therefore the hypothesis of this kind 
of action, not as involving in itself any manifest improbability, 
but as inconsistent with the existence of distinct approxima- 
* One of these sets of tensions may possibly modify the other, but in 
a general explanation, or in a first approximate calculation, this modifica- 
tion may be neglected. 
a 
