66 
REPORT— 1847* 
In these explanations the elevated mass may be supposed to have been 
fissured, either previously to the movement which we have been more espe¬ 
cially considering, or during that movement. If they were formed, as they 
might be, by the subsidence, and not during the elevation, those at A and 
A' (figs. 4, 5), and that at A (fig. 6) would commence at the surface, 
would be propagated downwards <iidy to a certain depth, beyond which 
the horizontal pressure would prevent their further progress. The fissure at 
S (figs. 4, 5j 6) would proceed from below upwards* till their propagation 
should be arrested in like manner by the horizontal pressure. In such 
cases, if a sufficient rjtiantity of matter should be subsequently carried awar 
by denuding agents, the external surface might be left without any trace of 
such fissures ns those at A and A’, but pr(^st>nting all the effects of intense 
horizontal pressure without vertical dislocation in the portions of the niaa 
n which those effects would be exhibited. 
It has already been intimated that it is necessary to suppose the lateral 
ma-sses respectively on the left of AN atid on the right of A'N' (figs. 4.5) 
to have been better supported at the instant of their highest elevation, 
than the intermediate portiotis; and the same supposition is ncccssarr m 
the case i-eprcsenteU In fig. 6, with reference to the mass on the letl of 
AN. It should be remarked, however, that the support required would only 
be necessary for the extremedy short time (a few minutes, or possiblyafcw 
Becuuds only) necoBsary to determine the maimer in which the subsidence 
would take place. This instantaneous support may probably be best re¬ 
ferred to accidcntnl causes. 
It may perhaps be thought that the aupposition of groat subsidence imme¬ 
diately consequent on great oh-volion, is not essential to account for the 
effects attributed to Biibsidt?nco in the preceiling explaiiulions, since these 
effects might be referred to tho direct action of the elevating force. But, 
in order thus to Recount for those effects, it would ficem necessary to sup¬ 
pose the elevating force to have acted with much greater intensity along 
certain lines than along other lines very near and parallel to them. Thus, 
lor example, in the case represented in fig. .'5, it would be necessary to 
make the supposition (or an equivalent one) that the force had acted with 
much greater intensity along the two anticlinal lines perpendicular to the 
plane of the section and passing respectively through A and A', than along 
the intermediate synclinal line through IM—an hypothesis so restrictive and 
arbitrary as to renilor any theory reposing on it of no value as a physical 
explanation of the phfenonicna of elevation. The explanation above given 
requires no such restrictive hy-potliesis, but merely assumes the instanta¬ 
neous influence of those local and irregular causes whieh must always be 
called into action in mechanical itpcrations like those we are considering. 
It should also be rpcollectcd, iJiat in tho theory more immediately before ns, 
the agency by which elevation has been usually produced, is supposed to 
have been that of an intuinescent fluid mass, a supposition so strongly 
tioncd by numerous observed pluenomcna, that no point of geological theory 
can, perhaps, be reganled as more completely establislu-d ; in such case, any 
law of linearity ill the action of the ♦ levalltig force, like that above niw 
tioned, would geem to be imposHilile. Moreover, the thcorv appeals to ex- 
isting.volcanic masses as nffonling by analogy the strongest roason to belief 
that the intumescenei' of tho Huiil ma'^s must have bcmi due to the elastic 
gases genemted wiihin it; and it nlso appeals, in support of the same con¬ 
clusion, to the- absence of any othrr cause hilherto aupirested, adequate to 
produce the intumosrcuce in question. Hut if these hypotheses be admitted, 
subsidence immediatoly after cousideiable and sudden elevation must be re- 
