HOLMES.] THE EAST GALLATIN MOUNTAINS. 19 



that this rock belongs in the conglomerates. In all other cases it ap- 

 pears in connection with the older strata. 



EAST GALLATIN MOUNTAINS. 



The range known by this name forms one of the most striking topo- 

 graphical features of the Park. Heretofore but little has been known 

 of its character or extent, and its geological structure was entirely un- 

 known. From nearly all parts of the Park its higher summits are dis- 

 tinctly visible. At the southern extremity is a naked, yellowish summit 

 (see sketch, Plate VIII), which is particularly conspicuous, and is plainly 

 visible from nearly all the flat country about the Madison Geyser Ba- 

 sins ; while at the northern extremity Electric Peak lifts its pyramidal 

 crest far above its fellows. The distance between these peaks is about 

 fifteen miles. At the extreme northern end the range terminates in Cin- 

 nabar Mountain. 



The continuation of this range to the westward and its connection 

 with the main Gallatin Kange are not well known. It is composed al- 

 most wholly of sedimentary rocks, the main range being chiefly vol- 

 canic. It has a decided north and south trend and is separated from 

 the main range by low passes. Its western slopes are drained chiefly 

 by the headwaters of the Gallatin River, and by the two large creeks 

 that eut«r the Yellowstone in the vicinity of the second canon. The 

 eastern face is drained by Cache Creek and Gardiner Eiver. 



The eastern and southern faces of the range are very abrupt and pre- 

 sent a very good section of the formations. The strata rise at a gentle 

 angle from the line of the Yellowstone fault at Cinnabar Mountain and 

 probably retain pretty nearly the position given them at the time of the 

 post-Cretaceous displacements. 



Beginning at Cinnabar Mountain and passing along the eastern base 

 of tfae range we find that group after group of the geologic series rise 

 from the level of the Park plateau and pass to the summits. At the 

 southern end of the range the granites appear in a crescent-shaped ex- 

 posure. 



There is at present no apparent reason for supposing that at the close 

 of the post-Cretaceous disturbance the whole series of formations was 

 not continuous to the east and west of this range. To the east we have in 

 Mount E varts proof of continuation in that direction ; however this may 

 have been, it is quite clear that at the time of the flooding of the Park 

 region with its great sheets of igneous matter, the eastern and southern 

 faces of this range rose as abruptly along the borders of the great basin 

 as now. In this basin the igneous matter was poured, and at the end of 

 the period of the rhyolitic flows a great undulating plateau was formed, 

 which, around the bases of this ancient fragment, had a general eleva- 

 tion of about 9,000 feet. 



In Plate IX, I present a panoramic view of this range as seen from 

 the southeast, looking across twenty miles of the igneous plateau. In 

 the foreground we have the newly-discovered Gibbon Geyser Basin 

 with its surrounding of dense forests. At the left is Mount Holmes, 

 with its smooth conical summit, surrounded by a number of subordinate 

 cones. At the south base, rising about 800 feet above the general level, 

 are the Archaen granites; above this and extending to the summit is a 

 heavy series of Silurian strata into which have been intruded great 

 sheets and laccolitic masses of compact hornblendic trachyte. The dip 

 of the Silurian strata is to the north at a gentle angle, as may be seen 

 in the drawing. 



