ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
157 
Assuring in nature do, to a certain extent in one set of directions only, viz. in those ortho- 
gonal to the lines of horizontal pressure, coincide with those assigned by Mr. Hopkins. 
But it further follows, admitting the relative plasticity of the less coherent formations of 
our earth’s crust and the sufficient power of these tangential pressures for crushing up the 
others, that not only elevation may result from their vertical resolution, but that conditions 
must arise where these pressures act through considerable depths of materials differing in 
nature or in resistance, or in both, so that, by other analogous resolutions into the vertical 
or more or less toward it but in the opposite sign (i. e. downwards), depressions and hollows 
of any sort of form are producible. 
35. Also that when these tangential and compressive forces, taken in a horizontal 
plane, make an angle in meeting greater or less (as may also happen in a vertical plane), 
then lateral displacements and all the phenomena of shifted strata or interrupted veins 
or faults can be produced. 
36. We can also see that this view of the origination of the elevatory forces which 
have raised up our vast mountain-chains at once assigns a true and adequate cause for a 
limitation to their possible height ; for as soon as the mass already forced up presses 
by gravity downwards upon the resolved vertical force e (fig. 1) to such an extent 
as to equal the resistance to crushing of the rocks beneath in the directions t and the 
further effect of the tangential pressures in those directions must be expended in crushing 
the rock between them to powder, where in some direction the solid can yield unequally, 
or in forcing off the material in some other direction than in the vertical. It is difficult 
to see how any limit is assignable to mountain altitude upon Mr. Hopkins’s hypothesis, 
unless by calling in other hypotheses for limiting the uplifting force itself, the intensity 
of which we cannot estimate in his case, inasmuch as its very nature is left unknown. 
37. If, then, in Pkevost’s notion we find an adequate and consistent theory of elevatory 
forces, it is the writer’s belief that in it also (when followed to its legitimate consequences 
in one direction) we possess the clue that shall ultimately lead us to an equally simple, 
adequate, and consistent theory of Vulcanicity as here about being unfolded. 
38. That the globe is hotter as we descend into it may be accepted as a fact, even from 
the very limited number of trustworthy experiments we yet possess ; for artesian wells 
and small borings are alone to be relied upon, mines and coal-pits giving (for reasons 
not necessary here to enlarge upon) unreliable results. 
39. And as we are certain that the geothermal couche of uniform annual temperature is 
everywhere above the existing temperature of the celestial spaces, so our planet must be 
a cooling globe ; and if so, in accordance with all we know of the materials of which it 
consists, a contracting globe. 
40. The rates of increment of temperature with depth, however, present great dis- 
cordance even after we have excluded cases (such as mines or coal-pits at work) likely to 
contain accidental sources of error. Not only does the rate vary from 1° Fahr. in 15 feet of 
depth to 1° in more than 200 (Mallet, 4 Neapolitan Earthquake Report,’ vol. ii. p. 310), 
but in places within a mile or two of each other, and in the same formation, it varies as 
much as from 208 to S3 feet for 1° Fahr. (op. cit.). Even in the same vertical line of 
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