ABSENCE OP PLICATION IN THE ROCKY SYSTEM. 47 



the Apalachians. It is certainly true that the study of these mountains 

 has not been so minutely detailed nor so long continued as that of mount- 

 ains situated in populous countries ; that a considerable portion of them 

 have not been examined geologically at all. But, on the one hand, the 

 number of which we already possess a preliminary knowledge is considera- 

 ble, and on the other hand the remarkable distinctness with which structural 

 facts are there displayed, and the comparative ease with which they may 

 be read, justify more confidence in our conclusions than might otherwise 

 have been admissible. No one familiar with the progress of knowledge in 

 this special direction can fail to recognize the conspicuous absence of plica- 

 tion in the mountain structures which are found east of the Sierra Nevada. 



Yet in some portions of this great expanse of territory there are im- 

 portant flexings and warpings of the strata. This is particularly true ot 

 the Basin Ranges. But a very significant distinction is necessary here. 

 These flexures are not, so far as can be discerned, associated with the build- 

 ing of the existing mountains in such a manner as to justify the inference 

 that the flexing and the rearing of the ranges are correlatively associated. 

 On the contrary, the flexures are in the main older than the mountains, and 

 the mountains were blocked out by faults from a platform which had been 

 plicated long before, and after the inequalities due to such pre-existing flex- 

 ures had been neaidy obliterated by erosion. It may well be that this ante- 

 rior curvation of the strata has been augmented and complicated by the 

 later orographic movements. But it is not impossible to disentangle the 

 distortions which ante-date the uplifting from the bending and warping- of 

 the strata which accompanied it, and it is only the latter that we can prop- 

 erly associate and correlate with the structures of the present ranges. These 

 present no analogy to what is usually understood by plication. The amount 

 of bending caused by the uplifting of the ranges is just enough to give the 

 range its general profile, and seldom anything more. The same fact is pre- 

 sented in the noble ranges of Colorado. Along their flanks the sedimentary 

 strata roll up usualty with a single sweep, and high on the slopes are cut off 

 by erosion. The typical anticlinal axis is not a characteristic feature of 

 the -Rocky Mountain system 



The type-section of the Park Mountains of Colorado, as given by the 



