February 96, 1897.] 



SCIENCE. 



329 



sediments ? Why, then, did it, from a pre- 

 vious land condition, ever commence to sub- 

 side ? And why, when the load was great- 

 est, namely, at the end of the Cretaceous, 

 did it begin to rise ? Again, from that time 

 to this it has risen 20,000 feet? Of this 

 about 12,000 feet have been removed by 

 erosion, leaving still 8,000 feet of elevation 

 remaining. Now if this elevation be the re- 

 sult of removal of weight by erosion, how 

 is it that a removal of 12,000 feet has caused 

 an elevation of 20,000 feet ? This result is 

 natural enough, however, if elevation was 

 the cause and erosion the effect, for the ef- 

 fect ought to lag behind the cause. It is 

 evident, then, that we must look elsewhere 

 — that is, in the interior of the earth — for 

 the fundamental cause, although, indeed, 

 the effects of this interior cause may be in- 

 creased and continued by the addition and 

 removal of weight. 



But perhaps the best illustration of the 

 distinctness of the two kinds of causes of 

 these movements is found in the oscillations 

 of the Quaternary period. I say best be- 

 cause in this case the effects of the two may 

 be disentangled and viewed separately, and 

 this in its turn is possible because the load- 

 ing in this case is not by mere transfer from 

 one place to another, and therefore is not 

 correlated with unloading. In fact, the 

 elevation in this case is associated with, 

 and in spite of, loading. The elevation, as 

 we all know, commenced in late Tertiary 

 and culminated in early Glacial. This ele- 

 vation was, at least, one cause, probably the 

 main cause, of the cold and the ice accumu- 

 lation, but the elevation continued in spite of 

 the accumulating load of ice. Finally, how- 

 ever, the accumulating load prevailed over 

 the elevating force and the previously ris- 

 ing area began to sink, but only because the 

 interior elevatory forces had commenced to 

 die out. Then with the sinking commenced 

 a moderation of the climate, melting of the 

 ice, removal of load, and consequent rising 



of the crust to the present condition, but 

 far below the previous elevated condition, 

 because the elevating forces, whatever these 

 were, had in the meantime exhausted them- 

 selves. If it had not been for the inter- 

 ference of the ice load, I suppose that in- 

 stead of the double oscillation which actually 

 occurred there would have been a simple 

 curve of elevation coming down again to 

 the present condition, but culminating a 

 little later and rising a little higher than 

 we actually find it did. 



The question arises as to how great an 

 area is necessary for the operation of the 

 principle of isostasy? What extent and 

 degree of inequality of surface may be up- 

 held by earth rigidity alone ? 



The recent transcontinental gravitation 

 determinations by Putnam and their inter- 

 pretation by Gilbert* seem to show a de- 

 gree of rigidity greater than previously sup- 

 posed. They seem to show that while the 

 whole continental arch is certainly sus- 

 tained by isostasy — that is, by deficiency 

 of density below the sea level in that part, 

 the continental area being lighter in pro- 

 portion as it is higher — yet great mountain 

 ranges like the Appalachian, Colorado and 

 Wasatch mountains show no such means of 

 support, but are bodily upheld by earth rigid- 

 ity ; and even great plateaus, like the Colo- 

 rado plateau, 275 miles across, are largely, 

 though not entirely, sustained in the same 



way. 



Monodinal Mountain Ranges. 



Until recently, mountain ranges were sup- 

 posed to be all made in one way, namely, by 

 lateral crushing and strata-folding and bulg- 

 ing along the line of yielding. To Gilbert is 

 due the credit of having first drawn atten- 

 tion to another type, conspicuously repre- 

 sented only in the Plateau and Basin region, 

 especially the latter — that is, those pro- 



* Gilbert: Phil. Soc. Washington, Vol. 13, 1895, p. 

 31. Gilbert: Jour. Geology, Vol. 3, 1895, p. 331. 

 O. Fisher: Nature, Vol. 52, 1895, p. 433. 



