ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
153 
This other cause, which occupies pages 52 and 53, is simply another hypothesis, that 
if the pressure of the superincumbent dome over the liquid lake be removed, partially 
or wholly, by its becoming a self-supporting dome or arch, then the relief from pressure 
thus arising must lower the fusing-point of the liquid material of the lake, and so keep 
it longer liquid. 
18. There is nothing absolutely in any of the known facts as to the materials of our 
earth’s crust to warrant our supposing isolated masses therein of far greater fusibility than 
the remainder, to say nothing of that material being ex necessitate penetrated by water 
from the surface, to which it is admittedly near ; for without the water we can have no 
volcano. Other serious difficulties occur as we attempt to follow out this notion by a 
comparison with observed facts of volcanic action at the vents, for which space cannot 
here be afforded. We pass on to some remarks on the theories of elevation. 
19. The words “elevation,” “upheaval,” “ Aushebung” “ soulevement” have been 
continually employed by the geologists of all countries, but especially of our own, in 
the loosest way, so far as the forming of any definite conception of the play of forces 
or mechanism of the movements is concerned. 
A wide survey of the writings of geologists proves, however, that the notion generally 
formed of a “ movement of elevation” is that it is produced somehow by a force acting 
beneath a limited area and in a vertical line, or nearly so, and in a direction from below 
upwards. Commonly it is assumed or inferred that the pressure upwards, beneath the 
area “ elevated,” has been that of a fluid, gaseous, as by Yon Bucii &c., or more or less 
perfectly liquid, as by most others. 
20. It is true that some geologists (amongst whom was the late Mr. Jukes) had some 
not very clearly defined notions that such machinery of elevation would not account for 
the facts as to elevated masses observable in nature, especially the frequent smallness of 
area in relation to the abruptness and height of the masses elevated. 
But that this notion, that elevation is produced somehow by nearly parallel forces 
acting radially to the spheroid, is even yet the one commonly maintained we need no 
proof beyond turning over the pages of the latest writers on geology. If we look at the 
figure on page 285 (2nd edit., 1862) of Scrope’s 4 Volcanoes,’ we shall see evidently that 
such is the notion he formed ; and, without citing further examples, it is that which 
Hopkins distinctly enunciates as his fundamental conception of the matter in his paper 
on “ Researches in Physical Geology ” (Trans. Cambridge Phil. Soc. vol. vi. part i.), 
where he says (p. 10) : — “ The hypotheses from which I set out, with respect to the 
action of the elevatory force, are, I conceive, as simple as the nature of the subject can 
admit of. I assume this force to act under portions of the earth’s crust of considerable 
extent at any assignable depth, either with uniform intensity at every point, or, in some 
cases, with a somewhat greater intensity at particular points — as, for instance, at points 
along the line of maximum elevation of an elevated range, or at other points where the 
actual phenomena seem to indicate a more than ordinary energy of this subterranean 
action. I suppose this elevatory force, whatever may be its origin, to act upon the lower 
