i8g 3 . THE GENESIS OF MOUNTAIN RANGES. 375 



radius, while the underlying real nucleus is a heated globe of some 

 7,500 miles in diameter, which has not parted with its original heat 

 or shrunk at all. The crust in compression is also found to be only 

 a few miles thick, the compression being most active at the surface, 

 gradually diminishing to nothing at the level-of-no-strain. These 

 conditions of strain and stress in a cooling globe being pretty 

 generally accepted by physicists of the day, Le Conte seeks to eke out 

 the amount of contraction required by the theory in other ways. 

 They are, however, so far, but formal assumptions having very little 

 foundation in physical facts, and may fairly be dismissed until some 

 better proof of their existence is forthcoming. 



Let us, however, grant the lateral movement required by the 

 formal theory, and consider how such machinery would be likely to 

 work. 



It is difficult to conceive in what way the lateral pressure of an 

 enormous shell, 8,000 miles in diameter, could be concentrated in one 

 or two places by any softening of strata under incipient mountain 

 ranges, for it would involve a torsion, in varying degrees, of the whole 

 crust, and a shearing over the whole area of the nucleus on which it 

 rests ; but this difficulty has been already dealt with by Dutton, so 

 we will not dwell upon it. When, however, we come to consider that 

 the application of the force is external to the lenticular plate of 

 sediments to be acted upon, our difficulties increase. A softening 

 of the crust along a certain axis might determine an elevation and 

 folding in its immediate vicinity, but it would hardly shade off into 

 those regular folds over a great area of country which are nearly 

 always present where former sedimentation and upheaval has taken 

 place. This would involve a refined gradation of softening that is 

 not likely to occur in nature. I have also in my work dwelt on the 

 fact that the pressure which has produced mountain ranges must 

 have been internal and centripetal, and have illustrated this idea by 

 reference to the domed form of anticlinals, varying from almost 

 a segment of a sphere through an ellipsoid to an inverted canoe- 

 form with their accompanying similarly-shaped but inverted 

 synclinals. 



It is, perhaps, only my individual opinion, but it seems to me 

 improbable that the earth's crust could, by hydrothermal softening, 

 be led into the symmetrical structures which characterise its surface 

 configuration. 



If, however, we look upon the pressure as internal to the folded 

 area, the difficulty vanishes. A heating of the strata and crust would 

 produce a state of compression acting throughout the mass from atom 

 to atom, and greatest where the heat is greatest. This, it appears 

 to me, is what is wanted to produce a folded mountain chain, and it 

 will, as I have attempted to show in detail in my work and various 

 papers, originate those great features of folds, foothills, and granitic 

 and gneissic axes distinguishing all great mountain chains. 



