118 J. LE CONTE — EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 



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 contraction 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 sur- 

 face 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 non-conductivity of these areas would prevent 

 the access of heat from below. Observation of lavas proves this. The 

 most vesicular and non-conductive 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 at- 

 tempt to carry out the idea into detail ; for the physical principles in- 

 volved, 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) 

 According to this view, the earth is supposed to have first solidified at 

 the center. This, on the whole, seems most probable. (2) The invest- 

 ing 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 superficial portions ; but since rocks contract in the act of 

 solidification,f 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 crust- 

 ing material would form the ocean areas. As in my view, therefore, the 

 oceanic areas are the denser and the land areas the lighter material. 



It is evident that, according to either view, but especially according to 

 mine, the material of the ocean basin areas down to the center of the 

 earth must be as much denser than the material of the land areas down 

 to the center as the subocean radii are shorter than the subcontinental 

 radii, and therefore that the two areas must be in perfect static equi- 

 librium with one another. Thus in the formation of continents the 

 claims of isostasy are completely satisfied. I say completely because 



*Bull. Geol. Soc. Am., vol. 4, 1893, p. 179. 



tKing and Barus. Am. Jour. Sci., vol. 45, 1893, P- r > 



