ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
209 
194. Taking now the earth’s mean diameter at 7912 
miles, two spherical 1 lines at right angles to each other, 
NcS and W rE, fig. 12, each of 180° chord, having a 
radial depth of 10 miles, and having no thickness at the 
extremities N S and WE, would at a width of only 255 
feet at the equator and pole respectively be equal in 
volume to the 7200 miles; so that a circumferential 
contraction on the surface of the earth, if all in two 
orthogonal great circles not exceeding 255 feet on about 
25,000 miles , is all we need to supply the volume of 
crushed rock within the first 10 miles of the surface, 
or vastly less if the contraction be extended down to a crust of 100 miles or 800 miles 
deep ; and this crushed rock is not subtracted from the earth’s volume, but simply 
transposed from beneath to its surface. 
These 400 volcanic cones probably do not very inadequately represent the totality of 
volcanic action that has taken place since the Tertiary or even a more remote epoch ; we 
know not how long that is ; but if we spread the volcanic action over even a few 
thousands of years, we see what an almost infinitesimally small amount of annual 
crushing by contraction is sufficient to account for the phenomena. 
It is not likely that future exploration will greatly add to the number of known 
volcanic cones, extinct, dormant, or active. Africa, the only great continent not yet toler- 
ably known, seems to contain very few and probably none in the interior; Borneo, 
New Guinea, and the Antarctic continent may have a few; but it is improbable that 10 
per cent, remain altogether to add to those already known. 
Again, it is highly improbable (on grounds winch it is impossible here to enlarge 
upon) that there are many submarine volcanoes, most probably none at all over the vast 
area of the bed of the deep ocean. One area alone has been discovered beneath the 
Atlantic Ocean, traversed as that has been for centuries now by ships continually. 
Some no doubt there are in shallow water, here and there, and more especially off 
the Pacific coasts of the American continent. 
However, let us grant “ scope and verge enough,” and assume that one half as many 
more volcanoes as those we know of remain to be added, or that their total is 600 in 
place of 400 ; the result will be that in place of 7200 cubic miles of mean crushed rock 
we shall require 10,800 cubic miles — a volume still perfectly insignificant when spread 
over the vast volume of the earth’s crust at 800 miles or even 100 miles thickness, and 
diffused in time over the unknown ages that have elapsed since the commencement 
of the existing forms of volcanic action. 
195. We may illustrate the matter in another way, taking for basis the best observed 
volcano in the world, Vesuvius, and estimating the annual vulcanicity of the whole 
globe by the scale afforded by its very numerous eruptions. 
Let it be assumed that the whole cone of Vesuvius above the level of the Hermitage, 
Fig. 12. 
