Transactions. — Geology. 



bear removal from the mould without warping, but whilst still 

 retaining a great degree of expansion from heat, it must be 

 taken from the mould and placed in position round the wheel to 

 which it is to form the tire. At first it is loose, but rapidly 

 tightens by contraction, as it gradually parts with its heat by 

 radiation, or, as is more usual, as it is cooled by the application 

 of douches of cold water. I am not aware to what extent the 

 density of fusible substances can be increased when in the solid 

 condition by the mere lowering of their temperatures, but I 

 doubt whether the density of any such substance, whatever 

 pressure it may be subjected to, (the heat generated by such 

 pressure being withdrawn,) can be thus increased so as to bring 

 it up to that which it possesses in the molten state. 



It will be seen, therefore, that in proportion to the heat to 

 which they are exposed, within the limits, in the descending 

 scale, of the lowest degree of temperature known to us on the 

 one hand, and the state of complete fusion on the other, metallic 

 and earthy fusible substances undergo three well-marked changes 

 in density — namely, they have a maximum when fused, a mmi- 

 mum when first solidified, and an intermediate density when 

 their heat in the solid condition is reduced by radiation. 



Now, assuming that the Earth was at one time in a molten 

 state, it is clear that so soon as it had parted with sufficient 

 heat to admit of the solidification of its outer surface, the 

 material so solidified would at once expand, and in course of 

 time would pass from the plastic to a rigid state. As radiation 

 proceeded further, the exposed surface would cool to such a 

 degree as to cause contraction of its substance, which would 

 then press with great force uj)ou the less rigid solid material 

 between it and the still molten mass below. But that molten 

 mass would still continue to part with its heat by conduction 

 and radiation, and its surface would solidify ; and, indeed, this 

 process would necessarily be continuous, until the rigid crust 

 had reached such a thickness as to oppose further sohdification. 

 Until this point had been reached, however, the consequences 

 of the processes to which I have referred would be to create 

 constant strains upon the contracted and still contracting outer 

 portions of the crust, and, as a result of such strains, the pro- 

 duction of fissures, or bulgiugs, or foldings, according to the 

 degree of rigidity to which it had attained. A further effect 

 would be to create cavernous spaces at various depths, and of 

 greater or less extent, into which masses of molten matter would 

 be injected by the pressure created upon the nucleus by the 

 plastic material interposed between it and the contracted outer 

 crust. Matter so injected would solidify with greater rapidity 

 than that which remained in general contact with the fluid mass, 

 and its expansion would certainly produce more violent action 

 on the surface of the globe than would result from the more 



