19"] REID— ISOSTASY AXD MOUNTAIN RANGES. 445 



There are great areas of the earth, such as the high plateau 

 regions in the west of the United States, where the rock has been 

 elevated many thousands of feet but without suffering any com- 

 pression whatever, which makes it quite evident that there are ver- 

 tical forces which produce many movements in the earth's crust. 

 Mr. Gilbert has given to these forces the name of cpcirogenic, or 

 continent-making forces, to distinguish them from orogenic forces; 

 but we must not forget that epeirogenic forces are apparently alone 

 active in the elevation of certain mountain ranges. The Sierra 

 Nevada, for instance, although its strata are much folded, owes 

 its present elevation to the vertical forces which seem still to be 

 tilting the great block. Alt. St. Elias also seems to have been tilted 

 up by vertical forces without any folding of its strata. 



The American geologists showed that a mountain range does 

 not rise haphazard in any part of the earth, but that it appears where 

 there was earlier a great geosynclinal, which had gradually sub- 

 sided and accumulated sediments to an extraordinary thickness, all 

 of them being laid down in comparatively shallow waters ; and it 

 was only after this preparatory step that the foldings and elevation 

 of the mountain range took place. 



But there is one important factor to which geologists have not 

 given proper attention, that is, the revelations of the plumb-line. 

 About the middle of the nineteenth century Archdeacon Pratt 

 pointed out that in the south of India the plumb-line was deflected 

 toward the Indian Ocean, and in the north of India, although it 

 was deflected somewhat toward the Himalaya mountains, still the 

 gravitational attraction of these mountains was considerablv less 

 than it should have been, if the density of the material in and under 

 them had been the same as in other parts of the earth's crust; and 

 he, therefore, suggested that the oceans were deep because the 

 material under them was heavy, and the mountains were high 

 because the material which composed them was light, and that in 

 general the amount of material under any two equal segments of 

 the earth was the same. But these facts did not make a great im- 

 pression upon geologists and did not prevent the further advocacy 

 of compression and the consequent accumulation of material as the 

 cause of mountain elevation. 



PROC. AMER. PHIL. SOC, L. 200 DD, PRINTED AUGUST 7, I9II. 



