GEOLOGICAL SURVEY OF THE TEREITORIES. 



67 



BANDED GNEISS 



out pretty clearly the story of tlie sedimentaiy formations of the West. 

 Along the valley of the Madison below the mouth of Cherry Creek, 

 there is a remarkable exhibition of the gneissic beds. For several 

 miles the strata are exposed so that the succession is perfectly clear for 

 thousands of feet in thickness. All the varieties of composition and the 

 flexures in the bedding peculiar to mining-rocks are seen in perfection. 

 Veins of feldspar and quartz extend across the bedding a foot or more 

 in thickness, evidently segregated in fissures like the mineral matter in 

 a lode. Masses of a very compact black horubleudic gneiss, 4 to G feet 

 thick, and 8 to 10 feet long, lie bet\Yeeu the strata as if they Avere old 

 intrusions of trap. I pj^ j„ 



have never seen a bet- 

 ter opportunity for a 

 detailed study of the 

 gneissic rocks, for they 

 are shown here in high 

 vertical walls for six 

 miles, inclining in the 

 same direction 30° to 

 50°. The beautiful ex- 

 amples of banded gneiss, 

 as shown in Fig. 17, are 

 not uncommon, and ex- 

 cite much attention. I 

 have used the term 

 gneiss in its broadest 

 sense, to signify granitic 

 rock composed of quartz, feldspar, and mica, arranged in well-de- 

 fined strata, usually thin, but sometimes reaching a thickness of several 

 feet. All the different forms which the metamorphic rocks assume are 

 noted in the catalogue of Br. Peale, appended to his report of this and last 

 year. In my report of last year I described quite minutely the basin 

 of the Three Forks. I then stated that the immediate valleys of the 

 streams near the junction had been carved out of the lake-deposits, and 

 on either side of the Madison Yalley, on the east side of the Jefferson, 

 and the west side of the Gallatin, the bluffs exposed fine sections of 

 these modern beds. From the courses of these great streams the source 

 of the waters during the lake-period might be read from the character 

 of the sediments. We find that they must have rushed with great 

 force along the valleys from the sources, after the impetus was gained 

 through the Upper Caiion, then through the Middle Caiiion, and begin- 

 ning to lose a portion of this force and moving along more quietly just 

 before entering the Lower Caiion, and when reaching the basin of the 

 Three Forks, the water must have been as quiet as in ordinary fresh- 

 water lakes of the i^resent day. The indications of swift currents are 

 not seen about the immediate sources of these streams, so that the sedi- 

 ments of the geyser-basins are comparatively fine, indicating moder- 

 ately quiet waters. All the other rivers tell pretty much the same 

 story. These sediments, made up as they are of the ditl'erent kinds of 

 rocks in the vicinity, are much mixed in their character. They are a 

 mixture of clay, sand, and marl, in varied proportion. Sometimes thin 

 indurated layers of clay will include several feet, then sand will pre- 

 dominate or sandstones in thin beds, or sort of indurated yellow or 

 cream-colored marl, and then, perhaps, a bed of loose gravel or pud- 

 ding-stone. There is a remarkable uniformity in the color and char- 

 acter of these sediments ail jover the West, from our north line to 



