GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 141 



highest ridges were covered with the Carboniferous limestones, which 

 I^assed down into some massive beds of quartzites before resting on the 

 gueissic granites. 



For ten miles from the Upper Willow Creek to the entrance of the 

 Boulder Creek into Jefferson Fork, we have the Carboniferous limestones 

 on our right, or west side ; on our left, or east side, basaltic rocks cover 

 the lake deposits. The valley is one to one and one-half miles wide, and 

 presents a grand display of the Pliocene marls and sands. The high 

 mountains, with the symmetrical cones, are also igneous. Between North 

 Boulder and Willow Creeks, a distance of about five miles, the Jefferson 

 Fork flows through one of the deepest limestone canons I have yet seen. 

 The walls on either side rise from 700 to 1,200 feet, almost vertically. 

 The general dip of all the limestones is northwest, and I estimated their 

 aggregate thickness at 2,000 feet. Masses of chert occur in the limestones, 

 which are filled with fossils, spirifers, corals, and crinoidal fragments. 

 The formations are much confused here, from the fact that the basalts 

 have been effused at a recent period, even since a large portion of the 

 lake deposits were laid down. In the gorges that lead down to the Jeffer- 

 son, they are exposed, and here and there are spread out over the marls. 

 Kow and then the limestones or older rocks crop out from beneath 

 them. Along the little streams that flow into the Jefferson as well as 

 the Jefferson itself, are distinctly marked terraces of the lake deposits, 

 for 50 to 200 feet above the river's bed. 



The North Boulder Creek enters the Jefferson from the north, through 

 a synclinal valley. On the west side the beds of limestone incline 

 northwest. The general trend of the synclinal is about northeast and 

 southwest. On the west side of the North Boulder and on the south 

 side of the Jefferson, the Carboniferous limestones prevail almost entirely. 

 There are only a few local outbursts of igneous rocks, and not occupy- 

 ing large areas. Above the canon the valley of the Jefferson expands 

 to a width of one and one-half miles. The lake deposits are again con- 

 spicuous, covering the entire valley and extending up the valleys of the 

 side-streams. About three miles above the mouth of the North Boulder 

 Creek, on the same side of the Jefferson, the ravines cut down into a 

 thick series of strata of sandstones, slates, clays, &c., which incline at a 

 moderate angle. These beds are, I think, local, and indicate volcanic 

 action connected with hot springs during the Phocene period. The 

 clays and sands are variegated, and thick beds of conglomerate occur. 

 The highest mountains are composed of quartzites and a group of light 

 gray vesicular strata in thin layers, which has somewhat the appearance 

 of igneous rocks. White alkaline efflorescence covers the surface in 

 many places. 



I may repeat again that the entire surface seems to have been wrinkled 

 or cramped into vast folds or ridges, with a general trend, nearly north 

 and south, or rather west of north and east of south; that the valleys of 

 the streams are for the most part synclinal depressions. The erosion 

 has been so great that it is quite uncommon for rocks of more mod- 

 ern date than the Carboniferous to be seen. The great valleys, or syn- 

 clinal depressions, during the latter Tertiary period were the basins oi 

 fresh-water lakes, so that we have everywhere the white and yellowish- 

 white sands, marls, clays, sandstones, and pudding-stones of the Plio- 

 cene lake deposits passing up into the Quaternary or local drift. It is 

 not uncommon for these modern lake deposits to be swept away, so 

 that we know of their former magnitude only by isolated remnants here 

 and there. During this lake period changes were going on in the sur- 

 face ; the general elevation of the country most probably continued, so 



