512 Notices of Memoirs — N. S. Shaler 



these facts we have to set those evidences of igneous action afforded 

 by volcanoes and associated phenomena, and which have, not without 

 reason, been supposed to give trustworthy evidence of a generally 

 fluid condition of the interior. Fairly weighed, however, all that 

 can be considered as proven by all the evidences we have is, that in 

 that portion of the past history of the earth of which we have 

 record, there has existed a condition of igneous fluidity beneath a 

 large part, if not the whole extent, of the surface. That this igneous 

 fluidity extends to the centre, or even that it is of more than a very 

 few miles in depth, are suppositions which derive no valid support 

 from igneous phenomena. The increase of temperature as we go 

 from the surface towards the centre, and the extreme elevation of 

 heat which must exist at considerable depths, can not be regarded 

 as evidence of the general fluidity, until it has been shown that the 

 internal pressure has not a greater influence in preventing lique- 

 faction, than internal heat in producing that condition. In the 

 present state of knowledge, or rather ignorance, of the physical 

 questions involved in this problem, the safest position is that which 

 conflicts least with the conclusions derived from the cognate sciences 

 of astronomy and physics. The former science protests that certain 

 observed facts could not exist if the mass of the earth was essentially 

 fluid, and that tried by tests far more unerring than any the 

 geologist is able to apply, the conclusion is reached that our planet 

 is at least as rigid as glass, and probably as rigid as steel. From 

 the physicist we hear that all the known materials which have come 

 to us from the earth's interior, contract in cooling, and that the 

 general internal fluidity would cause any crust to shatter to pieces 

 and fall in fragments into the fluid below, as soon as it had attained 

 any such thickness as we know the crust to have. If we attach to 

 these calculations the importance they deserve, we are forced to 

 admit that the idea of the igneous fluidity of the interior is quite 

 untenable. 



A much more satisfactory view than that just referred to, which 

 will not conflict with the results of investigations in the exact 

 sciences, may be obtained by a brief consideration of the possible 

 conditions of solidification of the cooling earth. If the effect of 

 pressure in promoting solidification at the earth's centre were 

 greater than the effect of heat in resisting solidification, then the 

 mass would congeal first at the centre, and solidification extend 

 thence towards the surface. If, on the other hand, the effect 

 of the pressure at the centre failed to overcome the tendency to 

 liquefaction induced by the extreme heat of that point, then we 

 must suppose that cooling went on until the whole mass was 

 reduced to something like an equal temperature throughout, and the 

 whole sphere became solid at once. During this process of cooling 

 down, successive crusts might be formed, but they would necessarily 

 be transient phenomena, breaking to pieces as soon as they began to 

 attain considerable thickness.^ 



^ Sec the Preliminary Observations to the paper of Hopkins above referred to, 

 where these considerations will be found. 



