MARVKE.] . GEOLOGY VALLEY OF THE MUDDY. 179 



ping east. The side valleys are in dark slaty shale with occasional are- 

 naceous and calcareous beds, reaching a thickness of about 350 or 400 

 feet. This outer ridge is capped with two strata of thiu bedded lime- 

 stone about 8 feet thick, separated by a thin shalier layer, all dipping 

 eastward at an angle, near the river, of 10°, flattening to 6° higher up. 

 The limestone varies from cofiee-brown to gray in color, is inclined to 

 saccharoidal, possesses a strong ijetroleum odor, and contains many im- 

 perfect fossils of a few Cretaceous species. The one occurring quite to 

 the exclusion of others is a small Inoceramus, which Mr. Meek informs 

 me closely resembles Inoceramus acutirostris (M. and H.) of Cretaceous 

 IsTo. 2 of the Nebraska section. Passing on down the river, there ap- 

 pears here and there beneath the terraced lake-beds forming its sides a 

 long series of dark argillaceous shales, which dip eastward at a very low 

 angle. 



The lake-beds, indeed, which here at first rise in a few low terraces, 

 then gradually merge into the adjacent bulky and irregularly contoured 

 hills, everywhere so obscured these beds that no good section was ob- 

 tained of tihem. There appeared to be no prominent beds among them, 

 however, and as the limestones last described are quite characteristic 

 and appear at other points, I have, for convenience, designated the hor- 

 izon as No. 3, though, as indicated by the fossil above, it may be lower 

 than No. 3, when paleontologically considered. If these beds are not 

 regarded as Cretaceous No. 3, there does not seem to be any horizon 

 here which could be so regarded. The shales above, which then become 

 No. 4, are probably thirteen or fourteen hundred feet thick. 



Just east of where the stream receives its main fork from the north, 

 rises the Upper Muddy Butte, a steep precipitous mountain reaching 

 some 2,400 feet above the river, and standing in its abruptness in strong 

 contrast with the surrounding country. Before reaching it, the hori- 

 zontal lake-beds which rest on the eastward dipping shales sweep north- 

 ward in low sloped hills, with many small ravines, to a series of hills 

 which are lava-capped, the lava having flowed down from the great 

 basaltic mass resting on the lignitic at the east. The lava-capped hills 

 which remain near the sources of the North Fork are not visible in the 

 section^ being hid by the Upper Mud'dy Butte, though five of the western 

 ones are seen. The Butte is also a mass of basaltic lava, the steep west 

 front being faced with many columns; planes of division cut through the 

 mass, dipping east of an angle of 25°, giving the impression that the 

 mass has been upturned into that position. This lava reaches lower 

 down than any of the other masses, as if it occupied a depression in the 

 former surface. On the south side of the stream, opposite the Butte, 

 are two or three connected, indefinite hills, all of the lake-beds, which 

 extend westward across a small terraced valley, and lap up on the ridge 

 of Cretaceous No. 3. Upon and about them many lava boulders are 

 found, often scoriaceous, and not well rounded, the remains, probably, 

 of a southward extension of the lava flow, the remnants of which cap the 

 hills of the north. It is this flow which would seem to have determined 

 the divide between the north and middle parks, which is here a gentle 

 swell, hardly worthy of the name of pass. Whether the northern lava 

 masses rest on lake-beds or on a westward extension of the lower lig- 

 nitic beds was not ascertained. The latter is quite possible, as the lake- 

 beds would have to reach unusually high to cover the whole pass, while 

 the lignitic beds appear to flatten in their northern extension, and may 

 swing around somewhat to the west. " 



Just south of the Butte a branch joins the main stream from the east, 

 up which the eastern end of section 2 was made. A little south of this 



