Mr. Hopkins’s Abstract of his Memoir on Physical Geology. 23% 
ent depths in different places, but also along the same vertical 
line, so that one shall be placed under another. It might, L 
conceive, be shown to be highly probable, if we should again 
recur to the hypothesis of the original fluidity of the globe, 
that the deeper cavities would in such case be the more ex- 
tensive. 
The immediate consequence of the elevatory force, as al- 
ready remarked, will be to produce eatension, and consequent 
tensions, in the elevated mass. Our first object must be to 
determine the directions of these tensions, for the purpose of 
ascertaining those of the resulting fissures. We shall after- 
wards consider the influence of the constitution of the elevated 
mass ; at present it is only necessary to regard it as admitting 
of a certain small extension without rupturing. 
I. For the greater simplicity let us first suppose the ele- 
vated mass to be of indefinite length, of uniform depth, and 
bounded laterally by two vertical parallel planes, beyond which 
the disturbance does not extend. Let A B B! A! bea section of 
the mass by a vertical plane perpendicular to the axis of ele- 
vation, A C B originally coinciding with A B; and let us also 
suppose that every such section is precisely similar and equal. 
Then it is manifest that there can be no extension perpendi- 
cular to these sections*, and that, consequently, the whole ex- 
tension must lie in directions perpendicular to the axis of 
elevation. Now let us conceive for a moment the elevated 
mass to consist merely of a very thin continuous lamina of it, 
ACB. Then it is evident that the extension, and therefore 
the tension, at any point, as C, in the section, must be in the 
direction T C 'T’ of a tangent to the curve line ABC. Let 
us now conceive another lamina, similar to the first, but with- 
out any adhesion to it, superposed upon it. It is clear that 
its extension, and, therefore, its tension, must be precisely the 
same as that of the first lamina, always supposing the original 
* The hypothesis of indefinite length in the elevation is equivalent to 
that of its being terminated by sections equal and similar to the one de- 
scribed in the text, so far as relates to’ the absence of longitudinal ex- 
tension. 
