10 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



rising upon the eroded edges of the Cretaceous sandstones and shales 

 which formed the border of the flat basin into which the molten nuiss 

 was poured. With the aid of a field-glass the details of the contact be- 

 tween the rhyolite and the Cretaceous strata can be pretty clearly made 

 out, but the curious white line, bordered by a ferruginous belt, which 

 occurs aloug the contact, must he examined more closely to be fully un- 

 derstood. 



North from the point at which the rhyolite terminates, the outline of 

 the mountain sweeps down iu a bold curve toward the Yellowstone. There 

 is no longer a capping of lava to sustain the shari^ly escarped edge, but 

 a series of remarkable sheets of dark basaltic lava perform that office. 

 Unlike the rhyolite, which has been flowed as a sheet upon the surface 

 of the Cretaceous strata, these basaltic masses are included between the 

 yielding strata, and if examined carefully may be seen to cross from 

 horizon to horizon, breaking through the beds and pushing them aside, 

 and bending and crushing them in a most remarkable manner. Down 

 on the slopes to the north other masses of intruded rock are exposed, 

 and beyond these the whole wall becomes so broken down and smooth 

 that a horseman can ascend with ease. Down by the river a secondary 

 wall occurs, and this, with an opposing wall on the west side, forms a 

 little eaiion through which the river flows. These strata, together with 

 all those visible iu the great wall, are Cretaceous. The dips all along are 

 from 5 to 15 degrees to the north as seen from the west side of the river. 

 There is also a dip to the east from the brink of the wall still more 

 marked, and a very little beyond the crest the strata come within the 

 influence of the abrupt fold of the great displacement which passes from 

 northwest to southeast through the mass of the mountain. The dip, 

 as we approach the axis of the fold, becomes vertical, and in jjlaces 

 passes beyond 90°. The strata composing the upper half of the great 

 wall are chiefly sandstones, generally rather soft, grayish and yellowish 

 in color. luterbedded with the sandstones are shales, clays, and lig- 

 nites. The lower part of the slope consists of dark-gray shales, beneath 

 which, in the creek bed, are the sandstones of the Dakota Grouj). 



On the 12th of September our party set out on its second trip, the 

 march of the day being from the springs to Junction Valley. Wishing 

 to see as much of Mount Evarts as possible, I determined to climb the 

 wall at the first accessible point toward the north, visit the basaltic 

 outcrops, and cross the flat summit of the mountain to the mouth of Black - 

 tail-deer Creek. Riding down the steep slopes over the old hot-spring 

 deposits and Cretaceous shales, I crossed the river about a mile below 

 the springs, and led my mule up the long, steep, and clifi'-brokeu slope. 

 Along the river there is a broad belt of sloping ground that forms a sort 

 of talus to the wall ; this space is covered with sage and shoit grass, and 

 is deeply cut by a multitude of narrow gulches which have been scored 

 out by avalanches from the steeper walls above. 



In ithe cliffs, from the base to the summits, the rocks which form the 

 principal outcrop are fine-grained, yellowish sandstones, often quite 

 massive. 



There are numerous beds of sandy shales, and not infrequently inter- 

 calated lamina of quartzitic sandstone. At the base some of the falling 

 masses contain indistinct impressions of deciduous leaves, together 

 with imperfectly i^reserved fruits and irregular masses of carbonaceous 

 matter. 



Throughout the section there are occasional indications of coal, which 

 is apparently an impure shaly lignite. The exposures are, however, far 

 from satisfactory. Outcrops of basalt occur, but are obscured by debris. 



