50 PROCEEDINGS OF THE PHILADELPHIA MEETING 



and are imposed on the otluM- two or primary movements as modifying, ob- 

 scuring, and often completely masking their effects." 



According U) iJaiia's coiircpLion, the solidification of tlie globe was first 

 at the center. The investing liquid arranged itself in layers of increas- 

 ing density downward from the surface to the solid nucleus. Certain 

 areas of the surface became crusted earlier than the general surface. 

 Becoming heavier by solidification than surrounding mass, the solid crust 

 sank, and was replaced by inflow of the lighter superficial fluid, which 

 in turn solidified and sank. The result was to build up from the solid 

 nucleus below a lighter, solid mass that constituted the primitive con- 

 tinent. The less rapidly crusting areas, of denser materials, formed the 

 oceanic areas. (Le Conte's Geological Society address, page 118.) Le 

 Conte's theory did not emphasize the conditions and effects of the super- 

 ficial crusting as did Dana's, but assumed area! differences in density and 

 iiiiiductivity. 



"If, then, over some large areas the matter of the earth were denser and 

 more conductive than over other large areas, the former areas, by reason of 

 thoir greater density alone, would sink below the mean level and form hollows; 

 for even in a solid — much more in a semi-liquid, as the earth was at that 

 time — there must have been static equilibrium (isostasy) between such large 

 areas. This would be the beginning of oceanic basins ; but the Inequalities 

 from this cause alone would probably be very small but for the concurrence 

 of another and much greater cause, viz., the greater conductivity of the same 

 areas. Conductivity Is not, indeed, strictly proportional to density ; but in a 

 general way it is so. It is certain, therefore, that the denser areas would be 

 also the more conductive, and therefore the more rapidly cooling and con- 

 tracting areas. This would increase, and in this case progressively increase 

 the depression of these areas. 



"The two causes — density and conductivity, isostasy and contraction — would 

 (•(incur, but the latter would be far the greater, because indefinitely cumulative. 

 The originally evenly spheroidal lithosphere would thus be deformed or dis- 

 torted, and the distortion, fixed by solidification, would be continually increased 

 until now." (Address, page 116.) 



This idea of Dana, that the continental areas were the first to solidify 

 and the oceanic areas subsequently, was accepted by Le Conte and applied 

 in harmony with his own thought as follows : 



". . . 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 con- 

 ductive 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, wliile the less conductive land areas would certainly be the first to 

 crust, because the non-conductivity of those areas would prevent the access 

 of heat from below." 



