126 J. LE CONTE — EARTH-CRUST MOVEMENTS AMD THEIR CAUSES. 



measurable at the present time, but for which no adequate cause has been 

 assigned and no tenable theory proposed; and (d) isostatic movements 

 or gravitative readjustments, by transfer of load from place to place, by 

 erosion and sedimentation, or else loading and unloading by ice accumu- 

 lation and removal, and also by readjustment of great crust blocks. If 

 the previous one (c) or oscillatory movements have masked and so ob- 

 scured the effects of (a) continent and ocean basin-making, this last (rf), 

 isostasy, has concealed the effects and obscured the interpretation of all 

 the others, but especially of (b and c) mountain-making forces and the 

 forces of oscillatory movements. In fact, in the minds of some recent 

 writers it has well-nigh monopolized the whole field of crust movements. 

 We shall not make secure progress until we keep these several kinds of 

 movements and their causes distinct in our minds. 



