1866.] 13 [Shaler. 



If this calculation is correct, (and that it Is in a g'eneral way cor- 

 rect, does not seem to admit of much doubt, provided we accept the 

 hypothesis of original igneous fluidity,) it follows that the gradual cool- 

 ing of the deeper portions of the crust must result in the formation of 

 a strong lateral pressure at every point nearer the surface. The truth 

 of this proposition is readily seen, when we consider that while the 

 original surface, which in ten thousand years after the hardening of the 

 crust had been reduced to the temperature of the atmosphere, re- 

 tained the same temperature in the ages which followed, the por- 

 tions of the ciiist beneath were constantly parting with their heat, 

 and approaching nearer to the thermal condition of the surface. 

 There would be no shrinkage of the surface layer from the loss of 

 heat, while from this cause the contraction of the deeper portions 

 would be considerable. This would give precisely the conditions 

 requisite to produce a rending and upfolding of the superficial strata 

 of the outer shell. Immediately after the formation of a crust, the 

 progressive diminution of the interior heat would begin to produce a 

 tension on the surface which would augment as the ceaseless flow of 

 heat went on, until either a rupture of the contracting beds, or the 

 folding together of the superficial layers, relieved tlie strain. Both 

 these methods of accomplishing the movement of contraction, have 

 been most probably operative at different times and places in the 

 earth's history. Furthermore, as the upper portions of the crust, or 

 region of slight contraction, is of much less thickness than the region 

 which, by its considerable contraction, produced the tension, we 

 would expect the fracture to take place on the surface, rather than 

 below. There is one thing which could operate to prevent the cer- 

 tain contortion of the superficial portions of the crust, and that is 

 the horizontal position of its beds ; as ordinarily constituted, the 

 resistance which the upper few miles of the crust could oppose to the 

 action of any force tending to throw it into folds, is very great. 

 AVhen the contortion has once begun, and this resistance fairly over- 

 come, all further changes would meet with comparativel} little 

 resistance. 



We have spoken only of those cases where the original surface had 

 continued to exist from the beginning, while the isogeothermals 

 beneath them had gradually sunk deeper and deeper towards tlie, 

 centre of the earth. This being a very unlikely condition, it remains 

 to be seen what would be the effect where the actions of denudation, 

 or deposition, are going on. It is evident that whenever the rate of 

 denudation was such that the removal of the crust took place with 

 the same rapidity as the recession of the isogeothermals from the sur- 

 face, there could be no lateral strain produced by the loss of heat. 

 Where, on the other hand, rapid deposition of materials was taking 



