Crosby.] 460 [November 7, 



earth's crust under the influence of the tangential thrust is in- 

 dorsed by Professor Dana, althought it appears to be inconsis- 

 tent with his theory of the origin of continents. As already 

 shown, he admits the existence of a nearly universal mobile layer; 

 but he also goes farther and insists that all important movements 

 of elevation and subsidence, except where mountains are formed, 

 are due to the bending of the crust in consequence of the lateral 

 pressure arising from the cooling and shrinking of the interior. 

 In other words, Professor Dana says that the crust experiences a 

 grand distortion before the strain becomes great enough to crush 

 it. 



The argument for this theory may be summed up as follows : 

 The earth either has or has not a plastic stratum. If it has not, 

 it is impossible to explain the phenomena of elevation and sub- 

 sidence and the formation of mountain systems; but if it has, the 

 continents and ocean-basins can only exist as upward and down- 

 ward bendings of the crust, and against this view of their consti- 

 tution no insuperable objections have been urged. 



2. The continents and ocean-basins are due to the 

 greater conductivity and more rapid cooling of the con- 

 tinental portions of the earth's crust ; hence they are per- 

 manent, and the continents are wider and the seas 

 deeper now than at any former period. 



This theory rests at the very outset upon an assumption which 

 is not supported by a vestige of evidence ; namely, that the earth 

 was originally, and is now, of unlike composition along different 

 radii or on different sides, the continental portions of the crust 

 being composed of denser materials than the oceanic. If the liq- 

 uid globe had possessed this constitution the ellipsoidal form of 

 the equatorial section which, under the most refined measure- 

 ments of geodesy almost disappears, would, in obedience to the 

 laws of hydrostatic equilibrium, be strongly marked. 



It seems strange that an assumption so vital to the theory 

 should have been made without any attempt to demonstrate its 

 validity. What are the facts that support it ? Where are the 

 analyses showing an essential unlikeness in composition between 

 the different portions of the earth's crust ? — showing, in other 

 words, that the rocks exhibited on the continents are denser, i. e., 

 more basic on the average, than those forming the ocean floor ? 



