OF VOLCANIC ROCKS. 5] 
Since the early day of the ingenious speculations of Descartes, this great and 
general cause has been considered as the main agency to which the disturbances 
on the surface of the globe are due; and, though having been more or less in 
favor at different times, the doctrine has at no time been completely abandoned. 
The various aspects which it has periodically assumed, the latitude given to it 
on one side, and the objections raised against it on the other, mark one feature 
of the phases of the gradual progress of science. It has been applied in different 
forms to explain the mode of origin of ancient eruptive rocks; and, since Dolo- 
mieu and, in a more elaborate way, his pupil Cordier, have assigned the same 
original source to voleanic activity, the contraction of the interior of the globe, by 
radiation of its heat into space, has been considered as offering a sufficient explana- 
tion for the majority of the phenomena which are often united under the term 
‘‘vuleanism.” But, giving all due consideration to the vast effects of which it has 
undoubtedly been the cause, the conception of the modus operandi of this agency 
(contraction) alone meets with considerable difficulty. An outward tension might, 
indeed, result in the formation of fractures on elevated places; and, supposing for a 
moment that these fractures would descend into regions where matter was in a liquid 
state, then the latter might possibly be ejected, and caused to accumulate on the sur- 
face. But such would hardly be the effect of an inward tension. It may, too, cause 
the rending of the crust and, possibly, the filling by liquid matter of those rents 
which are in the lowest places ; but it does not explain the extrusion of this matter 
to the surface, nor the fact of its particular accumulation on elevated parts of the 
same. It would take too much of our space fully to detail the numerous mechanical 
difficulties which occur, if one attempts to explain all the phenomena comprised in the 
name ‘‘ vuleanism” by the exclusive assumption of the contraction of the interior of 
the globe. Some of the more obvious objections against this theory will be briefly 
mentioned in the course of the following considerations. 
A number of facts point towards the existence of some unknown force below 
the earth’s solid crust, which counteracts in a considerable measure the permanent 
subsidence of the latter by contraction. It is perfectly evident that the secular rising 
of parts of the surface of the globe above the level of the sea cannot be merely the 
apparent effect of the different degree of its general subsidence, as has been maintained 
by very distinguished geologists; but that elevation, that is, the periodical increase of 
the distance of parts of the surface of the globe from its center, must be a reality. 
Considering the amplitudes of the changes of level that have taken place in historical 
time, they will be found to sum up to such figures as, if reduced altogether to subsi- 
dence, would indicate a far greater shortening of the radius of the globe within that 
time, than is compatible with astronomical calculation. It is true that the retardation 
of the rotation of the earth by the tidal wave must counteract, in some measure, the 
acceleration caused by any shortening of the radius of the planet ; but this retardation 
is insignificant if compared with the amount of the changes of level. The reality of 
elevation is forced more directly upon the mind, if those cases are taken into consider- 
ation where certain portions of continents are rising above the level of the sea at a 
more accelerated rate than neighboring regions, which is of very frequent occurrence 
(89) 
