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



primary movements as modifying, obscuring, and often even completely 

 masking their effects. This important point will be brought out as we 

 proceed. We will take up these movements successively in the order 

 indicated above. 



1. Ocean basin-making Movements. 



I have already given my views on this most fundamental question 

 very briefly in my " Elements of Geology," a little more fully in my first 

 paper, " Origin of Earth Features," * and in my memoir of Dana.f I 

 give it still more fully now. 



We may assume that the earth was at one time an incandescent, fused 

 spheroid of much greater dimensions than now, and that it gradually 

 cooled, solidified, and contracted to its present form, condition, and size. 

 Now if at the time of its solidification it had been perfectly homogeneous 

 in composition, in density, and in conductivity in every part, then the 

 cooling and contraction would have been equal on every radius, and it 

 would have retained its perfect, evenly spheroidal form ; but such abso- 

 lute homogeneity in all parts of so large a body would be in the last de- 

 gree improbable. 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 their 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 equi- 

 librium (wostasy) between such large areas. This would be the begin- 

 ning 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. Conduc- 

 tivity 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 again increase, and in this case progressively 

 increase the depression of these areas. The two causes — density and con- 

 ductivity, isostasy and contraction — would concur, but the latter would 

 be far the greater, because indefinitely cumulative. The originally evenly 

 spheroidal lithosphere would thus be deformed or distorted, and the dis- 

 tortion, fixed by solidification, would be continually increased until now. 

 When the earth cooled sufficiently to precipitate atmospheric vapor the 

 watery envelope thus formed would accumulate in the basins of the 

 lithosphere and form the oceans. It is possible and even probable that 



*Arn. Jour. Sci., 1872. 



f Bull. Geol. Soc. Am., vol. 7, 1895, pp. 461-474. 



