EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 235 



probably primitive movements by which oceanic basins and continental 

 masses were first differentiated and afterwards developed to their pres- 

 ent condition ; (2) those movemen ts by lateral thrust by which mountain 

 ranges were formed and continued to grow until balanced by exterior 

 erosive forces; (3) certain movements, often over large areas, but not 

 continuous in one direction, and therefore not indefinitely cumulative 

 like the two preceding, but oscillatory, first in one direction, then in 

 another, now upward and then downward; (4) movements by gravita- 

 tive readjustment, determined by transfer of load from one place to 

 another. Perhaps this last does not belong strictly to pure interior or 

 earth-derived forces, since the transfer of load is probably always by 

 exterior or sun-derived forces. Nevertheless they are so important as 

 modifying the effects of other movements, and have so important a bear- 

 ing on the interior condition of the earth that they can not be omitted 

 in this connection. 



Now of these four kinds and grades of movement the first two are 

 primary and continuous in the same direction, and therefore cumulative, 

 until balanced by leveling agencies. The other two, on the contrary, 

 are not necessarily continuous in the same direction, but oscillatory. 

 They are, moreover, secondary and are imposed on the other two or 

 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," 1 and in my memoir of Dana. 2 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 

 absolute homogeneity in all parts of so large a body would be in the 

 last degree 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 equilibrium (isostasy) between such large areas. This would 

 be the beginning of oceanic basins; but the inequalities from this cause 



'Am. Jour. Sci., 1872. 



2 Bull. Geol. Soc. Am., Vol. 7, 1895, pages 461-474. 



