Febeuaey 26, 1897.] 



SCIENCE. 



325 



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 conduc- 

 tive 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-conduc- 

 tive lavas are the soonest to crust, but for 

 that very reason the slowest to cool to great 

 depths. 



l>ro doubt many other objections may be 

 raised, especially if we attempt to carry out 

 the idea into detail; for the physical princi- 

 ples involved, and especially the conditions 

 under which they acted, are far too com- 

 plex and imperfectly understood to admit 

 of such detail. It is safest, therefore, to 

 confine ourselves to the most general state- 

 ment. 



It may be well to stop a moment to com- 

 pare 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 investing liquid, say 

 from 50 to 100 miles thick, might well be 

 supposed to arrange itself in layers of in- 

 creasing 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 



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

 t King and Barus. Am. Jour. Soi., Vol. 45, 1893, 

 p. 1. 



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. 



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 equilibrium with one an- 

 other. Thus in the formation of continents 

 the claims of isostasy are completely satis- 

 fied. I say completely because this is not 

 a partial equilibrium resisted by rigidity 

 but enforced by pressure; it is original and 

 without stress. 



2. Mountain-making Movements. 



I have so recently discussed this subject* 

 that I shall have little more to say now. 

 Mountain ranges are of two types, namely, 

 the anticlinal or typical and the monoclinal 

 or exceptional. The one are mountains of 

 folded structure, determined by lateral 

 thrust, the other of simpler structure and 

 determined by unequal settling of great crust 

 blocks. It is only of the former that I shall 

 speak now. The other or monoclinal type 

 will come up under another head. 



It will not be questioned that mountain 

 ranges of the first tj^pe are formed by lateral 

 thrust, however much we may difier as to 

 the cause of such thrust; nor will it be 

 questioned that they are permanent features 

 determined by continuous movement, how- 

 ever much they may be modified by other 

 kinds of movement or reduced or even de- 

 stroyed by subsequent erosion. I have 

 placed them, therefore, among the effects of 



* President's address, Am. Asso. Adv. Sci., Madi- 

 son meeting, 1893. 



