Crosby.] 456 [November 7, 



to the earth's rotation, this was the normal constitution of the 

 liquid globe ; yet, after solidification, this equal radial distribu- 

 tion of matter would very soon be spoiled by denudation ; and 

 now, after the continents have been repeatedly swept away and 

 the debris piled upon the sea-floor, we may fairly claim that it 

 has entirely ceased to exist. The horizontal movement of the 

 crust during mountain-making also operates to the same end ; for 

 there is certainly, as Professor Le Conte himself insists, more 

 matter along the radii terminating in the Sierra Nevada and 

 other mountain ranges now than before the mountains were 

 formed. 



The notion, then, that the terristrial radii are now of equal 

 weight is wholly untenable ; but the continents, with their vast 

 plateaus and mountain ranges are actual existences ; proving that 

 Le Conte has greatly over-estimated the levelling effect of gravi- 

 tation upon the inequalities of the earth's surface. 



Having shown that the well established facts of denudation and 

 mountain-making are a sufficient refutation of the second part of 

 Le Conte's objection to the theory here advocated, I turn now to 

 the consideration of the first part where he states, in effect, that 

 continental and oceanic arches resting on a plastic stratum could 

 not sustain themselves even though the crust were several hun- 

 dred miles thick. But, first, I will point out an apparent over- 

 sight in the only alternative which he allows those who believe in 

 a plastic stratum. He says, " If there be, indeed, a solid crust on 

 a liquid interior (or plastic zone), in order to sustain itself the in- 

 equalities of the upper surface in contact with the air must be 

 repeated on the lower surface / " and his figures in illustration 

 show inequalities on the under surface of the crust equal in form 

 and size to those on the upper surface. This equality between 

 the upward and downward protuberances of a floating crust 

 could exist only in the case where the density of the crust is just 

 half that of the supporting liquid ; but we are obliged to suppose 

 that the difference in specific gravity between the crust and liq- 

 uid in the case of the earth would be very small. If we assume 

 that the liquid is one-tenth heavier than the crust, then a protu- 

 berance of two miles on the upper surface of the crust would re- 

 quire a protuberance of twenty miles on the lower surface to sus- 

 tain it. Hence Le Conte is quite right in saying that this theory 



