1866.] 9 [Shaler 



into the contraction of the igneous rocks in cooling, make it impossi- 

 ,ble to conceive how a solid crust formed on a fluid interior could be 

 sustained, subjected as it has been to innumerable shocks, sufficient to 

 rupture it, and sink the fragments in the fluid below. Against these 

 facts we have to set those evidences of igneous action affbi'ded 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 temperatui'e 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 liquifaction, 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 Asti'onoray 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 mate- 

 rials which have come to us from the earth's interior, contract In cool- 

 ing, 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 attained by a brief consideration of the possible 

 conditions of solidification of the cooling earth. If the efiect 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 pres- 

 sure at the centre failed to overcome the tendency to liquefaction 



