EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 241 



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 mod- 

 ifying 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 consecpieut 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 continental 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 princifde 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 can not exist permanently. 

 The earth must gradually yield fluidally or plastically until static equi- 

 librium is established or nearly so. Thus continuous transfer of mate- 

 rial from one place to another by erosion and sedimentation must be 

 attended with 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 aud nearly all great mountain regions observable at 

 the present time. The same seems to have been true in all geological 

 times, for it is obviously impossible that 40,000 feet of sediments could 

 have accumulated in the Appalachian region in preparation for the 

 Appalachian's birth unless there were continuous pari passu subsidence 

 ever renewing the conditions of sedimentation. 



Now, there can be no doubt as to the value of this principle, but 

 there is much doubt as to the extent of its application. The operation 

 of exterior causes, such as transfer of load by erosion and sedimenta- 

 tion, are so comparatively simple and their effects so easily understood 

 that we are tempted to push them beyond their legitimate domain, 

 which in this case is to supplement and modify the more fundamental 

 movements derived from interior causes. We are thus tempted to gen- 

 eralize too hastily and to conclude that all subsidence is due to weight- 



1 Phil. Society of Washington, 1892. 

 sm 96 16 



