Febeuaet 26, 1897.] 



SCIENCE. 



323 



Volcanoes and earthquakes, although so 

 striking and conspicuous, are probably but 

 occasional accidents in the slow march of 

 these grander movements. It is only of 

 these last, therefore, that we shall now 

 speak. 



KINDS AND GRADES OF EARTH-CRUST MOVE- 

 MENTS. 



The movements of the earth's crust deter- 

 mined by interior forces are of four orders 

 of greatness : (1) Those greatest, most ex- 

 tensive, and probably primitive movements 

 by which oceanic basins and continental 

 masses were first differentiated and after- 

 ward developed to their present condition ; 

 (2) those movements 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 indefi- 

 nitely cumulative like the two preceding, 

 but oscillatory, first in one direction, then 

 in another, now upward and then down- 

 ward ; (4) movements by gravitative read- 

 justment, 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. N'evertheless they are so 

 important as modifying the effects of other 

 movements and have so important a bearing 

 on the interior condition of the earth that 

 they cannot 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 there- 

 fore cumulative, until balanced by leveling 

 agencies. The other two, on the contrary, 

 are not necessarily continuous in the same 

 direction, but oscillatory. They are, more- 

 over, secondary and are imposed on the 

 other two or primary movements as modi- 

 fying, 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-mahing 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 con- 

 tracted to its present form, condition and 

 size. Now if at the time of its solidification 

 it had been perfectly homogeneous in com- 

 position, 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 conduc- 

 tive 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 alone would probably be very 

 small but for the concurrence of another 

 and much greater cause, viz, the greater 

 conductivity of the same areas. Con- 

 ductivity is not, indeed, strictly propor- 

 tional to destiny ; but in a general way it 

 is so. It is certain, therefore, that the den- 

 ser areas would be also the more conduc- 



*Am. Jour. Sci., 1872. 



fBull. Geol. Soc. Am., Vol. 7, 1895, pp. 461-474. 



