52 Mr. J. Evans on Geological Changes in the [Mar. 15, 



there would, owing to the different specific gravity of the transported soil 

 and the displaced water, be a disturbance in the equilibrium of the crust, 

 and a consequent change in the position of its axis of rotation. 



Now if all this be true of a sphere, it will also, subject to certain 

 modifications, be true of a spheroid so slightly oblate as our globe. 



The main difference in the two cases is, that in a sphere the crust may 

 assume any position upon the nucleus without any alteration in its struc- 

 ture, while in the case of the movement of a spheroidal crust over a 

 similar spheroidal nucleus, every portion of its internal structure must 

 be more or less disturbed as the curvature at each point will be slightly 

 altered. 



The extent of the resistance to an alteration of position arising from this 

 cause will depend upon the oblateness of the spheroid and the thickness 

 and rigidity of the crust ; while the thicker the latter is, the less also will 

 be the proportionate effect of such elevations, subsidences, and denudations 

 as those with which we are acquainted. The question of friction upon the 

 nucleus is also one that would have to be considered, as the internal matter 

 though fluid might be viscous. 



It will of course be borne in mind that the elevations and depressions of 

 the surface of the globe are not, on the theory now under consideration, 

 regarded according to the proportion they bear to the earth's radius, but 

 according to their relation to the thickness of the earth's crust; and that, 

 even assuming Mr. Hopkins's extreme estimate to be true, yet elevations 

 or depressions, such as we know to have taken place, of 8000 or 10,000 

 feet, bear an appreciable ratio to the 800 or 1000 miles which he assigns 

 as the thickness of the earth's crust. 



It is, however, to be remarked that the extremely ingenious speculations 

 of Mr. Hopkins are based on the phenomena of precession and nutation, 

 and that if once the possibility of a change in the position of the axis of 

 rotation of the earth's crust be admitted, it is not improbable that the 

 value of some of the data upon which the calculations of these movements 

 are founded may be affected. 



The supposition of the thickness of the crust being so great seems also 

 not only entirely at variance with observed facts as to the increase of heat 

 on descending beneath the surface of the earth, but to have been felt by 

 Mr. Hopkins himself to offer such obstacles to any communication between 

 the surface of the globe and its interior, that he has had recourse to an 

 hypothesis of large spaces in the crust at no great depth from the surface, 

 and filled with easily- fusible materials, in order to account for volcanic and 

 other phenomena. 



But though it may be possible to account for volcanoes upon such an 

 assumption, yet, as already observed, the phenomena of elevation and 

 depression, such as we find to have taken place, and more especially the 

 existence of vast geological faults, cannot without enormous difficulty be 

 reconciled with such a theory. 



