OF VOLCANIC ROCKS. 51 



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 volcanic 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 

 "vulcanism." 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 " vulcanism" 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 



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