6 EEPOET UNITED STATES GEOLOGICAL SUEVET. 



iDg districts, have not been made out. It has a strike nearly northwest 

 and southeast. Followed to the southeast this strike would carry it 

 through between Mount Washburne and Amethyst Mountain, and be- 

 yond this through the great volcanic range east of the lake. To the 

 northwest it would pass beneath the volcanic mass of the main Gallatin 

 range, and continue in the direction of the lower caiion of the Madison. 

 This displacement, so I'ar as exposed, is by no means a simple fault. 

 The broken edges of the strata have become involved, and on the 

 dro])ped side are much dragged and crumpled. The amount of throw or 

 displacement at any one point is difiicult to determine, owing to the fact 

 that we were unable to cross to the north side of the river to make the 

 necessary observations of the ui)lifted side. The summits of the high 

 range to the north, as viewed from the south side of the river, are ap- 

 parently, in many cases, composed of Archoean rocks, which rise to the 

 height of 5,000 feet above the bed of the Yellowstone at Cinnabar Mount- 

 ain. In the vi{;inity of this mountain the top of the pre-Tertiary forma- 

 tion of the south side of the fault are depressed to the river level. By 

 adding 5,000 feet to the thickness of the pre-Tertiary strata we hav^e a 

 minimum estimate of the throw, at about 15,000 feet. 



In Section A I have represented as nearly as possible the condition of 

 the strata as seen at Cinnabar Mountain. The location of this section, 

 as well as of those which follow, is shown on the accompanying map, 

 Plate III. It will be seen that Cinnabar Mountain is formed of a series 

 of strata that has been folded abruptly up along the south side of the 

 fault, and that at e there has been an abrujit auxiliary displacement along 

 which the Cretaceous strata are much crumpled. The vertical beds of 

 the mountain comprise nearly the whole of the Mesozoic and Paleozoic 

 rocks. At e we have the Middle Cretaceous shales ; to the south of this 

 the strata are but slightly disturbed, and rise in a gentle slope of from 

 5° to 15° to the summit of Electric Peak. South of this summit, along 

 the crest of the Gallatin Eange, the whole series of sedimentary strata 

 rises gradually to the surface until in Mount Holmes, twenty miles south 

 of Cinnabar Mountain, we have the Silurian strata, a series of compact 

 limestones, which correspond in appearance to the vertical beds on the 

 banks of the Yellowstone at &, Section A. 



I^forth of the river, at g, the rocks appear to be chiefly metamorphic. 

 There are, however, many patches of volcanic products, some being frag- 

 mentary and others probably intrusive. 



The fragment of the older sedimentary strata which forms Cinnabar 

 Mountain, is the only one of importance exposed along the line of the 

 displacement. In ascending the river, from the point h of the section, 

 we follow an open valley along the east base of Cinnabar Mountain, and 

 at the Cache Creek crossing reach the line of the auxiliary fold shown 

 in Section A at e. 



On the accompanying map the strike of the ui)turned strata is indi- 

 cated by broken lines. The approximate line of the great displacement 

 is represented by a heavy-dotted line, and the auxiliary fold by a light 

 one. If the older strata which api^ear in Cinnabar Mountain occur on 

 the north side of the river they are so obscured by alluvial and vol- 

 canic deposits, or so jdanicl down by erosion, as to pass unnoticed. The 

 Cretaceous rocks that form the river banks are much obscured by al- 

 luvial deposits, but such outcrops as occur indicate a vertical position. 



As we recede from the river on the south side, the Cretaceous rocks 

 soon disappear beneath the heavy deposit of tufaceous marls and con- 

 glomerates (c and d, Section B), which have been deposited horizontally 

 upon the folded and eroded strata. The marls and the great series of 



