1883.] 459 [Crosby. 



and grows as that grows. Of course this distortion does not re- 

 lieve the crust from the lateral thrust; but, after warping, the 

 thrust is due, not to the partial action of gravity upon the whole 

 crust, but to the entire weight of the continental masses, or those 

 comparatively small portions of the crust lifted above the mean 

 level of the ocean-floor. 



The argument, then, shapes itself in this wise ; granting that 

 the earth contains a mobile layer, and that the crust is not 

 equally thick, rigid, and convex in all parts, it follows that as the 

 contraction of the nucleus goes on the crust experiences either a 

 general distortion or continuous crushing ; but we are obliged to re- 

 ject the last supposition, because geologists recognize in the earth's 

 history distinct mountain-forming epochs. A certain amount of 

 distortion, therefore, is inevitable ; and we have simply to con- 

 sider whether the existing relief of the continents could probably 

 be developed in this way before the strain would become great 

 enough to crush the crust. According to Mallet, if the crust 

 were completely separated from the nucleus, its sustaining power 

 would not exceed one four-hundredth of its own weight. Now the 

 continents, if we may estimate the thickness of the crust at fifty 

 miles and the continental relief at two miles and make proper al- 

 lowance for the higher specific gravity of the lower portions of 

 the crust, embrace about one one-hundred and fiftieth of the mass 

 of the earth's crust ; or a load two to three times greater than the 

 crust could sustain, if it were entirely unsupported. But it is 

 proper to assume that mountain ranges, being due to the horizon- 

 tal mashing of the crust, are in part sustained by corresponding 

 inequalities of the under surface of the crust. And besides there 

 is a vast difference between an arch supported only by its abut- 

 ments and one resting at all points upon matter which is only 

 imperfectly plastic, which is plastic perhaps only in comparison 

 with the enormous pressure to which it is subjected, and which 

 under some parts of the continents may be as rigid as the crust 

 itself. In view of these considerations it seems probable that the 

 continents approach, though they do not necessarily exceed, the 

 maximum load which the crust can sustain ; and it may be that 

 in the limited sustaining power of the crust we have a correct ex- 

 planation of the comparatively uniform and small depth of the 

 oceanic hollows. This view with regard to the behavior of the 



