1873-] Notices of Books. 249 



extant. Passing over those portions of his views which relate 

 to the period when our globe first liquefied from the nebulous 

 condition, and to the earliest stages of cooling by radiation, when 

 the crust was extremely thin, as also his account of the defor- 

 mation of the spheroid as one of the first effects of its con- 

 traction, we find that he has endeavoured to show that the rate 

 of contraction of the crust while very thin exceeded that of the 

 large fluid nucleus supporting it, and so gave rise to tangential 

 tensions in the crust, fracturing it into segments ; but next, 

 "that as the crust thickened, these tensions were gradually con- 

 verted into tangential pressures, the contraction of the nucleus 

 now beginning to exceed (for equal losses of heat) that of the 

 crust through which it cooled. At this stage these tangential 

 pressures gave rise to the chief elevations of mountain chains, — 

 not by liquid matter by any process being injected from beneath 

 vertically, but by such pressures mutually reacting along certain 

 lines, being resolved into the vertical, and forcing upwards more 

 or less of the crust itself. The great outlines of the mountain 

 ranges and the greater elevation of the land were designated 

 and formed during the long periods that elapsed in which the 

 continually increasing thickness of the crust remained such that 

 it was still, as a whole, flexible enough, or opposed sufficiently 

 little resistance to crushing to admit of the uprise of mountain 

 chains by resolved tangential pressures." " As our earth is still 

 a cooling body, and the crust, however, now thicker and more 

 rigid, is still incapable of sustaining the tangential pressures to 

 which it is now exposed, so it is by no means inferred that (re- 

 latively) slow and small movements of elevation and depression may 

 not still and now be going on upon the earth's surface ; in fact, 

 all the phenomena of elevation and depression, rending, &c, 

 which at a much remoter period acted upon a much grander and 

 more effeaive scale." " But the thickness of the earth's crust, 

 thus constantly added to, by accretion of solidifying matter from 

 the still liquid or pasty nucleus, as the whole mass has cooled, 

 has now assumed such a thickness as to be able to offer a too 

 considerable resistance to the tangential pressures to admit of 

 its giving way to any large extent by revolution upwards ; yet 

 the cooling of the whole mass is going on, and contraction 

 though unequal, both of thick crust and of hotter nucleus 

 beneath also, whether the latter be now liquid or not." " For 

 equal decrements of heat, or by the cooling in equal times, the 

 hotter nucleus contracts more than does its envelope of solid 

 matter. The result is now, as at all periods since the signs 

 changed of the tangential forces, thus brought into play, i.e., 

 since they became tangential pressures ; that the nucleus tends 

 to shrink away, as it were, from beneath the crust, and to leave 

 the latter, unsupported or but partially supported, as a spheroidal 

 dome above it." Mr. Mallet shows that, in this state of things, 

 and under the actual conditions to which the crust of the earth 



