1883.] 457 [Crosby. 



"breaks down with its own weight;" and it becomes doubly im- 

 portant for those believing in a plastic stratum to show that an 

 arched crust is a possibility. 



In the first place I desire to direct attention to the very slight 

 amount of distortion required in the crust to produce the existing 

 continents and oceanic hollows. Le Conte's diagram gives a 

 wrong impression, because the depressions are never concave. 

 Taking the mean height of the land at 1,000 feet and the mean 

 depth of the sea at 15,000 feet, we have a mean relief of 16,000 

 feet. So far as this argument is concerned, however, the oceans 

 are not so deep ; for the water helps to hold the depressed sur- 

 faces down and thus to maintain the distortion, being equivalent 

 in this respect to a layer of rock equal in thickness to two-fifths 

 of the depth of the ocean — 6,000 feet. This reduces the total 

 effective relief to 10,000 feet, or say two miles, which is one-two 

 thousandth of the earth's radius. In other words, the oceanic de- 

 pressions, taking the average depth, are not deeper relatively 

 than the varnish one-fiftieth of an inch thick on a globe eighty 

 inches in diameter. On such a model of the earth the continents 

 and ocean-basins would be produced by warping the film of var- 

 nish to the extent of its own thickness. 



Le Conte denies the possibility of crust-arches, even if the crust 

 is several hundred miles thick. But he admits, as all geologists 

 must, that the tangential strain due to the cooling and shrinking 

 of the earth's interior accumulates through long geological ages 

 before the crust finally finds relief by crushing. Of course no one 

 supposes now that this horizontal pressure ever even distantly 

 approaches its highest conceivable maximum, which would result 

 when an actual separation occurred between the earth's nucleus 

 and crust. But the attainable maximum pressure is, nevertheless, 

 very great ; being sufficient to crush a more or less rigid crust at 

 least twenty, and possibly fifty, miles in thickness. Now, the im- 

 portant question seems to be, has this enormous lateral thrust any 

 visible effect upon the crust before the time when the crushing of 

 the latter begins ? Those who answer in the negative must be 

 prepared to show that the crust is far more uniform in thickness, 

 weight, and rigidity than most geologists would probably be will- 

 ing to admit that it can be. The case is like this : by the shrink- 

 ing of the nucleus, the crust is left unsupported to a certain ex- 



