11)0 



therefore conclude the area embraced in the draina;,'e of the thivi^. ^aeat 

 braaches of the Missouri River— the Gallatin, Madison, and Jetferson 

 Eivers — is broken up by a largo mountain anticlinal. Itsentin; 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 com- 

 plex, from the introduction of a diflcrent 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 tlirown into the most 

 chaotic con fusion, so that the regular anticlinal ridges are, in many i)laces, 

 entirely broken up and lost. ^Sometimes we (ind 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 conceal- 

 ing and protecting from erosion the sedimentary beds. On the divide 

 between the Yellowst)ne Valley and the soiirci of the Gallatin, the 

 trachyte rocks cover all the sedimentary beds, even up to the Lignitic 

 inclusive; and here and there, wheri^ tiie mountaiu-streams hav(^ cut 

 deep gorges into the divide, the Silurian, Carbo:uferou>!, Tfias-;ic, Juras- 

 sic, Cretaceous, and Lignitic can be seen in their order of position . Not 

 unfreqiieutly all the older beds, w'ith the exception of theCarbojiifcrous, 

 had 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 Yellow- 

 stone Eiver, 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 fi?et of volcanic breccia and tuff's, 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 ieet in heiglit, apparently stratified and weathered so as to 

 present a peculiar but imperi'ect 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 be- 

 neath the breccia beds. From the entrance of Soda Butte Creek into 

 the East Fork of the Y^ellowstoue 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 Y'ellowstone National Park, the 

 Lignitic beds crop out here and there, from beneath the igneous covering, 

 in a baked or partially metamorphosed condition, containing fine .speci- 

 mens of plants of various si)ecies. 



There are also many examples of quite modern volcanic action, in which 

 the basalts have been poured over the modern Pleio(;ene or lake deposits, 

 and even the local drift. 



The evidence seems to be clear that the volcanic effusion conunenced 

 far back in the past, at the commencement of the elevation of the mount- 



