122 J. LE CONTE — EARTH-CRUST MOVEMENTS AND THEIR CRUSES. 



while others with more reason think that regions of heavy sedimentation 

 sink under the increasing load of accumulating sediments ; but it is evi- 

 dent that, while such theories may explain some local examples in vol- 

 canic regions and along some shorelines, they cannot explain subsidences 

 in the interior of continents, much less the wider and more extensive 

 movements spoken of above. We must look for some more general 

 cause. What is it? 



It must be confessed that the cause of these oscillatory movements is 

 the most inexplicable problem in geology. Not the slightest glimmer of 

 light has yet been shed on it. I bring forward the problem here, not to 

 solve it, for I confess my inability, but to differentiate it from other 

 problems, and especially to draw attention to these movements as modi- 

 fying the effects of movements of the first kind, and often so greatly 

 modifying them as to obscure the principle of the permanency of oceanic 

 basins and continental areas, and even to cause many to deny its truth. 

 Nearly all the changes in physical geography in geological times, with 

 their consequent changes in climate and in the character and distribu- 

 tion of organic forms — in fact, nearly all the details of the history of the 

 earth — have been determined by these oscillatory movements ; but amid 

 all these oscillatory changes, sometimes of enormous amount and extent, 

 it is believed that the places of the deep oceanic basins and of the conti- 

 nental masses, being determined by other and more primary causes, 

 have remained substantially the same. 



4. Movements by gravitative Readjustments — Isostasy. 



This very important principle which, though partially recognized by 

 Herschell, was first clearly enunciated by Major Dutton under the name 

 isostasy.* The principle may be briefly stated thus : In so large a mass as 

 the earth whether liquid- within or solid throughout, it matters not excess 

 or deficit of weight over large areas cannot exist permanently. The earth 

 must gradually yield fluidally or plastically until static equilibrium is 

 established or nearly so. Thus continuous transfer of material from one 

 place to another by erosion and sedimentation must be attended w T ith 

 sinking of the crust in the loaded and rising of the crust in the unloaded 

 area. -In this way we may account for the sinking of the crust at the 

 mouths of great rivers and the correlative rising of interior plateaus 

 and nearly all great mountain regions observable at the present time. 

 The same seems to have been true in all geological times, for it is obvi- 

 ously impossible that 40,000 feet of sediments could have accumulated 



*Phil. Society of Washington, 1892. 



