OCR Text |
Show ABSENCE OF 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 mountains 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 considerable, 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 plication in the mountain structures which are found east of the Sierra Nevada. Yet in some portions of this great expanse of territory there are important 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 building of the existing mountains in such a manner as to justify the inference that the flexing and the rearing of the ranges are correctively 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 flexures had beL nearly obliterated by erosion. It may well be that this anterior 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 properly 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 presented in the noble ranges of Colorado. Along their flanks the sedimentary strata roll up usually 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 |