166 Notices of Memoirs — D. Forbes, F.R.S., 



bulging out at the equator, being that which a plastic mass reYolving 

 round its own axis would assume, is generally regarded by natural 

 philosophers as all but conclusive evidence that the earth, at an 

 early period of its history, must have been in a fluid condition. 



Although the doctrine that the earth is a molten sphere sur- 

 rounded by a thin crust of solid matter, was all but universally 

 taught by geologists, there have of late years been brought forward 

 several arguments to the contrary, which are apparently more in 

 favour of its being a solid, or nearly solid mass throughout, and these 

 arguments are fully entitled to our mature consideration. As our 

 object is not to defend any particular theory, but to arrive as nearly 

 as we can at the truth, we shall in the first place proceed to scruti- 

 nize all which has been brought forward in opposition to the older 

 hypothesis of the earth's internal fluidity, and then to consider 

 whether any other explanation as yet advaineed may be more in ac- 

 cordance with the facts of the case. 



First of all we have to answer the question as to whether it is 

 possible for such a thin crust to remain solid, and not to become at 

 once melted up and absorbed into the much greater mass of the 

 molten matter beneath it. This would doubtless be the case if the 

 central fluid mass had any means in itself of keeping up its high 

 temperature independently of the amount of heat which it actually 

 possessed when it originally assumed the form of an igneous globe. 

 This question, however, in reality, answers itself in the negative, 

 since it is self-evident that no crust could even commence to form 

 upon the surface, unless the sphere itself was at the moment actually 

 giving off more of its heat from its outer surface to the surrounding 

 atmosphere than it could supply from its more central parts in order 

 to keep the whole in a perfectly fluid condition, so that once such a 

 crust, however thin, had formed upon the surface, it is self-evident 

 that it could not again becorae melted up or re-absorbed into the 

 fluid mass below. 



The process of solidification due to external refrigeration would 

 then continue going on, from the outside, inwards, until a thickness of 

 crust had been attained sufficient to arrest or neutralize (owing to its 

 bad conductibility of heat) both the cooling action of the surround- 

 ing air and the loss of more heat from the molten mass within, and 

 thus a stage would soon be arrived at, where both these actions 

 would so counterbalance one another, that the further cooling down 

 of the earth would be all but arrested, a condition apparently ruling 

 at the present time, since the surface of the earth at this moment, so 

 far from receiving any or more than a miniite amount of heat from 

 the interior, appears to depend entirely, as regards its temperature, 

 upon the heat which it receives from the sun's rays. 



We have next to consider the argument, that if the earth's exterior 

 was in reality onily such a thin covering or crust like the shell of an 

 ^gg, to which it has often been comjoared, that such a thickness would 

 be altogether insufficient to give to it that stability which we know 

 it to possess, and that it consequently could never sustain the enor- 

 mous weight of its mountain ranges, such as, for example, the 



