1883.] 451 [Crosby. 



of this theory is that mountains are due to a horizontal mashing 

 up of the earth's crust along lines of weakness, the crushing force 

 being simply the tangential thrust induced in the crust by the 

 continued cooling and contraction of the earth's interior, after 

 the crust has attained a constant temperature and volume. Now, 

 according to Le Conte, this horizontal crushing of the crust, as 

 evidenced by folds and slaty cleavage, is so great that on the 

 average the breadth of the crushed area — the niountairj-zone — 

 is diminished in the ratio of 5 to 2 ; 10,000 feet becoming 4,000 

 feet, and 10 miles becoming 4 miles. In other words, the for- 

 mation of a mountain-range is equivalent, in one sense, to closing 

 a fissure several miles in width ; and this requires a decided hori- 

 zontal movement in a very large part of the earth's crust, the 

 latitude and longitude of places being permanently changed. 



This slipping of the crust over the nucleus could not take 

 place without a plastic zone, which is thus seen to be essential to 

 the generally accepted theory of the origin of mountains. 



Therefore, to sum up, whether we consider the relative distri- 

 bution of temperature, pressure and moisture in the earth's crust, 

 the phenomena of elevation and subsidence, the formation of 

 mountains, or the origin of volcanic products, to which I have 

 scarcely alluded, we equally reach the conclusion that the earth 

 must contain a nearly universal plastic stratum. Consequently, 

 any theory of the origin and relations of continents and ocean- 

 basins incompatible with the existence of this mobile layer must, 

 in the present condition of the science, be considered as in so far 

 inadequate. A yielding layer or bed, we are compelled to be- 

 lieve, underlies the continents and seas and must be regarded as 

 fundamental in any true theory of these grandest of the earth's 

 surface features. 1 



1 Within a few months several prominent geologists have revived the old view that 

 below a thin, solid shell the earth is wholly liquid. So far as the relations of conti- 

 nents and ocean-basins are concerned, the condition of the central portions of the earth 

 is of little consequence to one who has accepted the view here insisted upon, that the 

 crust rests upon a nearly continuous mobile layer; for the theory of a liquid globe 

 merely extends the mobile layer to the centre. It does not appear to the writer, how- 

 ever, that any arguments yet advanced forbid us to believe in the essential solidity of 

 the earth's great central nucleus. Hon. J. W. Powell, Director of the United States 

 Geological Survey, holds (Science, in, 480) that the phenomena of faulting, plication 

 and vulcanism are incompatible with a solid earth. But certainly a mobile layer ten 

 to twenty miles thick would satisfy the dynamical geologist as well as one 4,000 miles 



