426 



University of California. 



[Vol. i. 



ceded by the advocates of rigidity under load. The latter are 

 required to account for (i) the coincidence of a zone of subsidence 

 running parallel with the continental margin, and (2) the final 

 upheaval of these sediments into a mountain range. This makes a 

 rather strong case for the upholders of isostasy, but not, it is 

 believed, an impregnable one. It would be far stronger were conti- 

 nents invariably bordered by great mountain ranges, but such is by 

 no means the case, as can be seen by reference to any good atlas of 

 the world. Irregularities comparable to mountain chains exist, 

 moreover, as we know, in the deep oceans, far from any existing 

 land-mass. Then, too, there are not wanting eminent geologists 

 who deny that the process of mountain-building is anything more 

 than a very superficial one, not essentially involving the solid crust. 

 Thus Reyer,* as a preliminary to presenting his gliding theory, 

 refers to cases near Christiania, and in the Wesergeberge, where the 

 folded strata rests upon basements which have not shared in their 

 corrugation. 



Without accepting Reyer's extreme view, it does seem that the 

 effect of heavy accumulations of sediment upon the underlying crust 

 by the rise of the isogeotherms, has been frequently exaggerated. 

 It is quite commonly assumed that their rise softens the crust and 

 metamorphoses the lower sediments; but, in fact, we find no signs of 

 such metamorphism even in the oldest sediments, no matter how 

 deeply buried, as long as they have escaped vigorous crumplings 

 and igneous intrusions. 



The actual elevation of the mountain ranges is a thing difficult 

 to reconcile with the idea of delicate isostasy ; for if strata spread 

 out in a broad sedimentary lens are competent to depress the 

 earth's crust, how can the same sediments, when compressed with 

 no loss of mass to a smaller base, maintain their elevation above the 

 geoid ? They could do so if a large proportion of the sediments 

 (supposedly lighter than the underlying material) subsided to a 

 sufficient extent to buoy up the projecting mountain-mass above 

 them. But such a structure, as far as the writer is aware, has 

 never been recognized. Mountains in many cases reveal the base- 



* Ursachen der Dwformationen unci der Gebirgsbildung. Leipzig, 1892, 

 P- 23- 



