.324 11. F. RBID ISOSTACY AND EARTH MOVEMENTS 



gravity during the time of compression, we can gain an idea of it only 

 through geological observations which distinguish between folding and 

 uplift. Finally, the uplift of the underlying mass raises the region into 

 a true mountain range. 13 



There is nothing in the principle of isostasy that militates against the 

 coexistence of the expansion and consequent elevation and the folding, 

 but geological observations show that the elevation, now existing in the 

 cases cited above, has occurred at a distinctly later date than the folding. 

 The interval between them has been long enough, in some cases, to allow 

 a peneplanation of the surface ; in other cases depression to well below 

 sealevel has occurred and considerable thicknesses of sediments have been 

 deposited in this interval. 



Isostasy and Peneplanation 



It has been argued that during a state of peneplanation there has been 

 no movement of the area, which was being gradually worn down by ero- 

 sion, and therefore if it were in isostatic equilibrium before the erosion, 

 it could not be afterward. But this is begging the question. According 

 to the principle of isostasy, there must be, during the erosion, an influx 

 of matter below (with an unknown lag), to keep up the equilibrium. 

 This would cause a slow elevation of the region, but at a rate less than 

 the lowering by erosion, because the inflowing matter would have a 

 greater density than the surface matter removed. The lag could not be 

 very great, for regions now undergoing active erosion do not show a ma- 

 terial defect or excess of gravity. 14 I know nothing that renders this 

 I joint of view unsound. 



Isostasy and the Eigidity of Rock 



The argument that rock under pressure is too rigid to permit the move- 

 ments called for by the principle of isostasy is based on the excellent 



13 The conviction that folding and elevation are merely different aspects of the same 

 event has become so ingrained that the two words are often used almost as synonyms, 

 and it is sometimes difficult to decide whether a writer is recording a true uplift or 

 merely folding. It is very important that field geologists should distinguish between the 

 two processes. 



Dr. Rollin T. Chamberlin has attempted to determine the depth to which compression 

 has extended on the assumption that the elevation was entirely due to compression 

 (Jour. Geol., 1910, vol. xviii, pp. 228-251; and 1919, vol. xxvii, pp. 225-251) ; but if the 

 ideas of the present paper are right, his assumptions are wrong. He considered the 

 possibility of simple vertical uplift, but decided that it did not have an important part 

 in the elevations he measured. I do not think his observations justify his conclusions, 

 for I have been unable to make the details of the movements he deduces fit his general 

 idea. , 



14 S. G. Burrard: Geog. Jour., 1921, vol. lviii, p. 215. 



