Crosby.] 458 [November 7, 



tent ; but it will continue to maintain its original form of fluid 

 equilibrium, and the strain will be without visible effect until it 

 becomes great enough to crush the crust ; provided that the ten- 

 dency to follow the contracting interior is equal in all parts of the 

 crust ; i. e., provided that the thickness and rigidity of the crust 

 are sensibly the same at all points. The accepted theory of moun- 

 tain-formation, however, not to mention other considerations, for- 

 bids us to believe that the crust is thus homogeneous. But if it 

 is not, then it must, in the case supposed, be in a state of unstable 

 equilibrium ; the thicker and heavier parts are pulled downwards 

 most strongly ; and the result is a grand distortion or warping of 

 the crust whereby its interior capacity is diminished so that it can 

 accommodate itself to the shrunken nucleus. The depressed 

 areas become sea-floors and the elevated areas continents. It is 

 not clear how those who admit the existence of a plastic stratum 

 in the earth can consistently doubt that a condition of unstable 

 equilibrium in, and consequent warping of, the crust would in- 

 evitably follow the shrinking of the nucleus. 1 Of course, if the 

 earth is continuously rigid from centre to circumference, as Le 

 Conte claims, there is an end at once of all extensive elevation 

 and subsidence, and of all mountain-making too. For, although 

 Le Conte has not said it, yet it seems to be a logical consequence 

 of his theory that the crushing of the crust would not be limited 

 to narrow zones since that implies a slipping of the crust over the 

 nucleus which would be impossible if the two were everywhere 

 rigidly connected; but the crushing would be, like the wrinkling 

 of the skin of a withered apple, more or less uniformly distributed, 

 producing many minor corrugations but no lofty, well-defined 

 and dominant ranges. 



As already stated the warping of the crust, unlike the crushing, 

 does not require that the tangential pressure shall first accumu- 

 late through long periods of geological time ; but we may fairly 

 suppose that it begins soon after the development of the pressure 



l This point is easily illustrated by experiment. If from the interior of a thin sphe- 

 rical shell of some flexible substance we remove a portion of the air so that the atmos- 

 pheric pressure, which may be taken to represent gravity in the case of the earth, will 

 be greatest on the external surface and thus develop a tangential pressure in the shell, 

 it will be readily seen that it would be almost impossible to construct a model of such 

 perfect uniformity and symmetry that a general distortion or flattening of its form 

 would not be the immediate result. 



