ON THE THEORIES OP ELEVATION AND EARTHQUAKES. 47 
incrustation of the surface could take place so long as any inferior portion 
of the mass retained its perfect fluidity, because as the superior particles 
should become condensed they would continually descend into the perfect 
8 uid boueath, always supposing the mass in that state in which an increase 
of specific gravity would result from a decrease of temperature. ’I'he pro¬ 
cess of circulation would thus go on till every part of the tnaNj sboiikl have 
lost that degree of more perfecl fluidity, wliich admits of a motion of tlm 
^rticles among themselves being excited by their unetjual refrigeration. 
The circulation, therefore, would cease uearly contemporaneously in every 
part of the mass, which would then begin to cool by coculuetion, ra[iidly at 
the Burfacc exposed to the low tempeiature of the planetary space, and ex¬ 
tremely slow in the central parts, on account of the biuuII conductive power 
of the matter composing the earth. Consequently the globe would consist, 
after a oerlaiu time, of an exterior solid crust, and interior fluid matter, of 
which the fluidity would increase in approaching the centre, where it miaht 
Bbll approach to that more |>crfect fluidity which admiu of cooling by con¬ 
vection. With rererenoe, however, to the ineelmnicai action of any forcea 
pr^ucing either motion or hydrostatic pressure in the interior mass, the 
whole ot it might, as an approximation, be conahlored perfectly fluid. No 
attempt has yet been mude to determine the present probable llucktieas of 
the earths crust, assuming' it to have been originally in a state of fluidity, 
on account of tlie difficulty already mentioned, arising from our Ignorance 
of the influence of high temperature in resisting solidification, rompared 
with that qf great pressure in promoting it. All tlmt has hitherto been 
determined on the subject is, that the present state of the earthV surface 
may be consistent with thci existence nf a solid crust, of wliich the thickness 
IB small compared with the earth's radius. 
^ ‘ Let us now recur to the other case above mentioned, that in which the 
increase of pressure in (lescending towards the centre of the mas* is supposed 
0 lave a greater eflect in promoting soiidiflcatioii than the increaao of tem¬ 
perature in preventing it. Supposing the mass to have been firtl in a state 
in w ic/i every part was cooling by oonvcclion, this process would first 
cease, and that o( cooling by conduction begin at the centre, while the supe- 
iior portion would still continue to cool by convection, so that these two 
processes would for a time be going on simultaneously in different parts of 
e mass. It U manifest, however, that the central portion, cooling by con - 
uction, would constantly increase, while the exterior portion, cooling by 
convecUon, would constantly rliniinish, »o that at length no part of the mass 
wou be cooling by the latter jiroces*. IluforR it should reach this stage of 
ic re ngeratioii, the central portion of a mass so large a* the earth might 
pcome perfectly solid, so tlmt at the instant wlieo the circulation should 
en irely cease, tlie whole might consist of a solid central nucleus, surrounded 
t>y the external portion still in a state of fusion, and of wbicli the fluidity 
would vary continuously from tl.e solidity of the nucleus to tlie fluidity of 
be surface, where, at the instant we are sneakinir of, it would be ju»t such 
bB not to admit of circulation. 
“ \y ben llie mass should have arrived at this stage of the cooling, a change 
wonhl take place in the process of solidification, which it is important to 
remark. The superficial parts of ihe juass must in all cases cool the most 
^pidly. and now (in consequence of the imperfect fluidity) being no longer 
b e to descend, a cm.'if will 1 m> formed on the surface, from which the pro- 
of solidification will proceed far more rapidly downwards, than upwards 
Bolid nucleus. Conscqueotly. then, our globe would arrive at that 
8 e, according to tlie mode of cooling we are now considering, in which it 
