328 



SCIENCE. 



[N. S. Vol. V. No. 113. 



these oscillatory movements is the most in- 

 explicable 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 modifying the effects of move- 

 ments 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 distribution 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 de- 

 termined 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 with sink- 

 ing 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 



* Phil. Society of "Washington, 1892. 



correlative rising of interior plateaus and 

 nearly all great mountain regions observa- 

 ble 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 pC''''i passu subsidence ever 

 renewing the conditions of sedimenta- 

 tion. 



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 legiti- 

 mate domain, which in this case is to sup- 

 plement and modify the more fundamental 

 movements derived from interior causes. 

 We are thus tempted to generalize too 

 hastily and to conclude that all subsidence 

 is due to weighting and all elevation to re- 

 moval of weight. Probably this is a true 

 cause, but not the main cause of such 

 movements. Doubtless the proposition is 

 true, but its converse is even much more 

 so. It is certain that thick sediments 

 may cause subsidence, but it is much more 

 certain that subsidence, however deter- 

 mined , will cause continuous sedimentation 

 by ever renewing the conditions of sedi- 

 mentation. It is true that removal of 

 weight by erosion will cause elevation, but 

 it is more certain that elevation is the cause 

 of removal of matter by erosion. 



Take again the Plateau region as an ex- 

 ample. We have seen that during the 

 whole Carboniferous, Permian, Triassic, 

 Jurassic and Cretaceous times this region 

 was subsiding, until at the end of the Creta- 

 ceous the earth's crust here had bent down- 

 ward 12,000 or 15,000 feet. Shall we say 

 it went down under the increasing load of 



