EVIDENCES OF ISO-STATIC EQUILIBRIUM 319 



present time, the world is everywhere in a fair degree of isostatic equilib- 

 rium. And, when we consider that this equilibrium holds in regions of 

 active erosion and in regions of active deposition, the conclusion is in- 

 evitable that as the equilibrium is disturbed it is readjusted by subter- 

 ranean flow, and that the earth is sufficiently yielding to permit this flow. 



Objections to former isostatic Equilibrium 



If the earth does yield in this way, it follows that the earth has always 

 been in isostatic equilibrium. Some geologists will not admit this; they 

 contend that we are now in a period of mountain-building and of great 

 disturbance, which permit the adjustment, but that this was not true in 

 former geologic times. So far as I know, the objection is based on the 

 contradiction to two hypotheses and not on the opposition to facts. The 

 hypotheses are : ( 1 ) that mountains have been raised by the compression 

 of the strata, and thus extra matter has been squeezed into them, and (2) 

 that the earth is too rigid to yield to forces developed by the transfer of 

 surface material. 



Vertical uplift of Mountain Ranges 



The study of the folds and faults and other characteristics of moun- 

 tains led the geologists of the last century to replace the earlier ideas of 

 von Buch, that mountains were suddenly uplifted by volcanic forces, with 

 the idea that mountains were squeezed up by tangential pressure. It can 

 be shown, however, from purely geological observations that many of the 

 most important mountain ranges of the world owe their elevation, not to 

 lateral compression, but to vertical uplift. 



The Sierras of California suffered strong compression and close folding 

 in pre-Cretaceous times. After this they were reduced to a peneplain 

 which could not have been very much above sealevel, and in quite recent 

 times they have been elevated, leaving the peneplain as a gentle slope to 

 the west, but without any additional folding. On the east the mountains 

 are bounded by a great and steep normal fault. It is evident that normal 

 faults can not be formed by pressure alone, and I have shown 8 that steep 

 faults, normal or reverse, require the action of vertical forces. It is clear 

 that the present elevation of the Sierras is not due to compression, but is 

 due to vertical force acting long after the folding occurred. 



The history of the Appalachian and the Rocky Mountains is very much 

 like that of the Sierras. The Appalachians were strongly folded and 

 faulted at the end of the Carboniferous. In the Cretaceous they had been 



8 In a paper on the dynamics of faulting, soon to be published. 



