OCR Text |
Show 209 their way directly through mountain- ranges instead of following synclinal depressions indicates that they began the process of erosion at the time of the commencement of the elevation of the surface. This is shown all along the valley of the Yellowstone, and more conspicuously in the valleys of the Madison and Gallatin, which have carved immense canons or gorges directly through two of the loftiest ranges of mountains in Montana. We believe that the course of these streams was marked out at or near the close of the Cretaceous period; and as the ranges of mountains were in process of elevation to their present height, the erosion of the channels continued. The details of the observations which contributed to form this opinion would occupy a chapter or two. I would be glad to find the leisure at some future period to extend these observations in detail all over the country west of the Mississippi, so far as it has beeu explored. It should be done by some one; but as these notes are intended simply to convey brief descriptions of the sections accompa-ujiug them, it cannot be done at this time. The sections, however, vitt, I think, be acknowledged by all geologists and geographers to convey marvelously true pictures of the surface features of a remarkably interesting region. J* |