Changes in the Position of the Axis of the Earth's Crust. 539 



Directly, however, that we argue from the contrary assumption, 

 that the solid portion of the globe consists of a comparatively thin 

 but to some extent rigid crust, with a fluid nucleus of incandes- 

 cent mineral matter within, and that this crust, from various causes, 

 is liable to changes disturbing its equilibrium, it becomes apparent 

 that such disturbances may lead, if not to a change in the position 

 of the general axis of the globe, yet at all events to a change in the 

 relative positions of the solid crust and the fluid nucleus, and in con- 

 sequence to a change in the axis of rotation, so far as the former is 

 concerned. 



The existence in the centre of the globe of a mass of matter fluid 

 by heat, though accepted as a fact by many (if not most) geologists, 

 has no doubt been called in question by some, and among them a 

 few of great eminence. The gradual increase of temperature, how- 

 ever, which is found to take place as we descend beneath the surface 

 of the earth, and which has been observed in mines and deep 

 borings all over the world, the existence of hot springs, some of 

 the temperature of boiling water, and the traces of volcanic action, 

 either extinct or still in operation, which occur in all parts of the globe, 

 afford strong arguments in favour of the hypothesis of central heat. 



And though we are at present unacquainted with the exact law 

 of the increment of heat at different depths, and though, no doubt, 

 under enormous pressure the temperature of the fusing-point of all 

 substances may be considerably raised, yet the fact of the heat in- 

 creasing with the depth from the surface seems so well established 

 that it is highly probable that at a certain depth such a degree of 

 heat must be attained as would reduce all mineral matter with which 

 we are acquainted into a state of fusion. When once this point was 

 attained, it seems probable that there would be no very great variation 

 in the temperature of the internal mass ; but whether the whole is 

 in one uniform state of fluidity, or whether there is a mass of solid 

 matter in the centre of the fluid nucleus, are questions which do not 

 affect the hypothesis about to be considered. 



Those who are inclined to regard the earth as a solid or nearly 

 solid mass throughout, consider that many volcanic phenomena may 

 be accounted for on the chemical theory, which has received the sup- 

 port, among others, of Sir Charles Lyell. But apart from the consi- 

 deration that such chemical action must of necessity be limited in its 

 duration, the existence of local seas of fluid matter, resulting from 

 the heat generated by intense chemical action, would hardly account 

 for the increase of heat at great depths in places remote from volcanic 

 centres ; and the rapid transmission of shocks of earthquakes and the 

 enormous amount of upheaval and subsidence as evidenced by the 

 thickness of the sedimentary strata, seem inconsistent either with the 

 general solidity of the globe or any very great thickness of its crust. 



The supposition that the gradual oscillations of the surface of the 

 earth, of which we have evidence all over the world as having taken 

 place ever since the formation of the earliest known strata up to the 

 present time, are due to the alternate inflation by gas and the subse- 

 quent depletion of certain vast bladdery cavities in the crust of the 

 earth, can hardly be generally accepted. 



