| OCR Text |
Show 199 therefore conclude the area embraced in the drainage of the three great branches of the Missouri River- the Gallatin, Madison, and Jefferson Sivers- is broken np by a large mouutain anticlinal. Its entire extent is not yet known. It probably covers a somewhat larger area, reaching over into the Yellowstone drainage, and extending north below the three forks. The structure of this area, although apparently so simple, is very complex, from the introduction of a different force, which seems to have acted synchronously with the one that lifted the granitic nucleus, and which may have been produced by it. This is the volcanic force which has acted most powerfully all over the region. It is by the effusion of the igneous rocks that the sedimentary beds are often thrown into the most chaotic confusion,- so that the regular anticlinal ridges are, in many places, entirely broken up and lost. Sometimes we find these beds in patches on the summits of the highest mountain- ranges, and again in the lowest valleys. Faulting on a large scale has been everywhere produced. Over large areas, also, the igneous rocks have been poured out, thus concealing and protecting from erosion the sedimentary beds. On the divide between tbe Yellowstone Valley and the sourca of the Gallatin, the trachyte rocks cover all the sedimentary bads, eveu up to the Lignitic inclusive; and here and there,' where the lnouiitaiu- streuiH have cut deep gorges into the divide, the Silurian, Carboniferous, Trias sic, Jurassic, Cretaceous, and Lignitic can be seen in their order of position. Not unfreqaently all the older beds, with the exception of the Carboniferous, bad been worn away prior to the effusion of the trachyte, and the beds of limestone crop out along the margins of the mountains and canons from beneath their igneous covering. There is a striking example of this along the East Fork of the Yellowstone River, as is shown in Plate X in the " View looking up Soda Butte Creek from Camp on the East Fork of the Yellowstone." We have here 2,500 to 3,000 feet of volcanic breccia and tuffs, mingled here and there with* trachyte. . This is cut in every direction by little branches into the most picturesque gorges, with very vertical walls on either side, from 1,500 to 2,500 teet in height, apparently stratified and weathered so as to present a peculiar but imperfect columnar appearance. The surface-weathering is in the form of immense castles, battlements, steeples, and towers. Index and Pilot Peaks, in the illustrations in the Annual Keport for 1872, page 47, convey a faint idea of the wilderness of unique forms which are presented to the eye over this great volcanic area. This volcanic material was laid on an irregular surface of limestone mostly of Carboniferous age. Sometimes the red- beds are seen just beneath the breccia beds. From the entrance of Soda Butte Creek into the East Fork of the Yellowstone to its source, a distance of ten miles or more, the limestones crop out on either side of the gorge. Sometimes only ten or fifteen feet are exposed, then again several hundred feet in thickness are seen, with only a thin bed of breccia on the summit Over a considerable' portion of the Yellowstone National Park, the Lignitic beds crop out here and there, from beneath the igneous covering, in a baked or partially metamorphosed condition, containing fine specimens of plants of various species. There are also many examples of quite modern volcanic action, in which the basalts have been poured over the modern Pleiocene or lake deposits, and even the local drift. The evidence seems to be clear that the volcanic effusion commenced far back in the past, at the commencement of the elevation of the mount- |
