EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 237 



present condition. It is true the inequalities are more accentuated in 

 some places, especially on the margins of the continental areas; but 

 this is due to another cause, mountain making, to be taken up later. 



Another objection will doubtless occur to the thoughtful geologist. 

 It would seem at first sight on this view that ocean areas cooling most 

 rapidly ought to be the first to form a solid crust, and the crust (if 

 there be any interior liquid still remaining) ought to be thickest, and 

 therefore least subject to volcanic activity, there; but, on the contrary, 

 we find that it is just in these areas that volcanoes are most abundant 

 and active. It is for this reason that Dana believed that laud areas 

 were the first and ocean areas the last to crust over. This is probably 

 true; but a little reflection will show that these two facts — namely, the 

 earlier crusting of the land areas and the more rapid cooling and con- 

 traction of the ocean areas — are not inconsistent with one another ; for 

 the more conductive and rapidly-cooling areas would really be the last 

 to crust, because surface solidification would be delayed by the easy 

 transference of heat from below, while the less conductive land areas 

 would certainly be the first to crust, because the nonconductivity of 

 these areas would prevent the access of heat from below. Observa- 

 tion of lavas proves this. The most vesicular and nonconductive 

 lavas are the soonest to crust, but for that very reason the slowest to 

 cool to great depths. 



No doubt many other objections may be raised, especially if we 

 attempt to carry out the idea into detail; for the physical principles 

 involved, and especially the conditions under which they acted, are far 

 too complex and imperfectly understood to admit of such detail. It is 

 safest, therefore, to confine ourselves to the most general statement. 



It may be well to stop a moment to compare with the above view that 

 of Dana, as interpreted and clearly presented by Gilbert in 1893. 1 (1) 

 According to this view the earth is supposed to have at first solidi- 

 fied at the center. This, on the whole, seems most probable. (2) The 

 investing liquid, say from 50 to 100 miles thick, might well be supposed 

 to arrange itself in layers of increasing density from the surface to the 

 solid nucleus. Now suppose for any cause, less conductivity or other, 

 certain areas crusted on the surface. These crusts would, of course, 

 consist of the lighter sujjerficial portions; but since rocks contract 

 in the act of solidification, 2 these solidified crusts would sink to the 

 nucleus and be replaced by similar lighter material flowing in from the 

 surrounding surface, which in turn would solidify and sink. Thus 

 would be built up from the nucleus below a solid mass consisting only 

 of the superficial, lighter material, to form the land, while the denser 

 and less rapidly crusting material would form the ocean areas. As in 

 my view, therefore, the oceanic areas are the denser and the land areas 

 the lighter material. 



1 Bull. Geol. Soc. Am., vol. 4, 1893, page 179. 



2 King and Barus. Am. Jour. Sci., vol. 45, 1893, page 1. 



