FOLDS AND FAULTS. 289 



than the restoration of the eroded beds in their present position would give, 

 if it had not been for the counteracting effect of faulting ; and faulting 

 might, in this sense, be considered a result of subsidence or of radial con- 

 traction. It is easily seen, for instance, by studying any one of the given 

 cross-sections of the Mosquito range, that were the movements of the faults 

 reversed, so as to bring the beds on either side of each back into their orig- 

 inal position, and thus leave them as they would have been if influenced 

 by plication alone, the range would have been about four thousand feet 

 higher than at present, supposing erosion to have acted under those condi- 

 tions with the same energy that it has under the present. This view of the 

 elevation of the range involves, it is true, a subsidence of the region ad- 

 joining the Sawatch shore line and probably of the whole Sawatch mass. 

 Subsidence and elevation in cases like this, which refer to a far distant 

 period of the earth's history and where limited areas are involved, are more 

 or less interchangeable terms, since the only fixed point to which they can 

 be related is the .center of the earth, whose distance cannot be determined 

 with a possible error less than the amount of movement involved, and we 

 have to content ourselves with the assumption that there must be a tend- 

 ency in all movements of the earth's crust to preserve a certain equilibrium, 

 and that, where one portion of the crust has been elevated in relation to an 

 adjoining one, the apparent movement is probably the sum of an actual 

 elevatory movement on the one side and of a subsiding movement on the 

 other, each of which is necessarily less in amount than the apparent move- 

 ment. 



The same may be said of areas large enough to assume an almost con- 

 tinental importance. Thus the Plateau region of the Colorado River has 

 evidently subsided relatively to the adjoining mountain areas of the Wa- 

 satch and of the Rocky Mountains, as shown by the great average differ- 

 ence of level of corresponding formations in these areas; but the Plateau 

 region must always have been relatively lower than these, since it was 

 from the abrasion of their land masses that its sediments were in large 

 measure derived. It may, however, be considered in general to represent 

 an area of subsidence, and the others to be areas of elevation; and the type 

 structure which prevails there, namely, that of broad level blocks descend- 



MON XII 19 



