190 GEOLOGICAL SUEVEY OF THE TEREITOEIES. 



here, therefore, a great fault, wbicli separates the sedimentary rocks of 

 the Blue River and Williams Mountains from the metamorphic rocks 

 of the peak and the region at the east, the down-throw being on the 

 western side. 



The sedimentary rocks which form the valley and lower portion of the 

 peak cannot be taken at less than nearly 6,000 feet thick, probably much 

 more, while the schists of the peak rise more than a thousand feet higher, 

 so that, making no allowance for an unknown thickness of material 

 eroded from the summit of the peak itself, the western side of the fault 

 must have moved down, with respect to the eastern side^ a distance of at 

 least 7,000 feet. 



This great fault passes northward east of the Williams Mountains, 

 but was observed nowhere along the Grand, and probably dies out in 

 that direction. Southward it appears to form the eastern side of the 

 valley of the Blue for some distance, while it may be the northern con- 

 tinuation of some Of the great faults that occur in the neighborhood of 

 Mount Lincoln, but the connection was not traced out at all. 



The beds which make up the west side of the peak are in part the 

 same as those of the Williams Mountains. At the base, but best ex- 

 posed on the north side of Pass Creek, or in the south end of the Wil- 

 liams Mountains, are the series of somber, dull-brown sandstones, ex- 

 posed in several heavier beds, with narrower, slatier beds between, 

 which lie all along at the base of the range. Those here exposed seem 

 to be the upper i)ortion of this zone. It was in those beds that the 

 fossils allied to Inoceramus harahini, (Morton,) before referred to, were 

 found, confirming the age of this horizon as being Cretaceous ISTo. 5. 



Above follows a slope of shalier beds, perhaps 500 feet thick, capped 

 with the bed of sandstones which forms the upper prominent layer seen 

 along the west face of the Williams Mountains. The characters of this 

 sandstone I seem to have failed to record, except that the main massive 

 portion was about 80 feet thick. Above were 500 feet, mostly shaly, 

 with another harder band of thin-bedded, dull-brown sandstone, about 

 40 feet thick on the summit. In the slope of about 600 feet of softer 

 beds, lying still above, some black argillaceous shale was observed, and 

 at the top harder sandstones, some shaly and dull-brown as before, and 

 some white, inclined to saccharoidal, a few of the harder beds being 18 

 inches thick. No fossils were observed. Some of the sandstones noticed 

 were whitish and reddish, and rather coarse, while a few only seemed 

 l)lainly composed of debris of metamorphic or granitic rocks, and to re- 

 semble therein the characteristic coarse ligni tic sandstones of the other 

 sections. Indeed, the series seems more to belong to the lignitic hori- 

 zon because lying above the usual thickness and divisions of the Creta- 

 ceous rocks, rather than by the close lithological resemblances so well 

 marked elsewhere in the park. Though differing from the Cretaceous 

 in lithological characters, and having no place in the usual Cretaceous 

 series of the park, they yet retain but to a slight degree the characters 

 of the lignitic rocks farther north. I have considered them the same, 

 however, and have so represented them on the maps and sections. 

 Search would undoubtedly be rewarded with fossils proving their age. 



From the base of the high terraced front of Ute Peak the long, low, 

 terraced lake-beds sweep out to the Blue, close to the west side of which 

 are the lower indefinite masses of morainal matter from the Blue Eiver 

 Mountains, partially confused with the lake-beds. A few miles to the 

 south the terraces are broken by a broad, uneven rise lying directly 

 across the valley, and through which the river flows in a caiion. Ap- 

 proaching it, it is found composed of the Cretaceous beds, the harder 



