STRUCTURE OF THE VALLEY DEPOSITS 279 



creek, a distance of about 2 miles. Tn the angle of its declivity, its gen- 

 eral form, and the character of the material next the surface it is essen- 

 tially like the other benches of the Cordilleran valley. It has the 

 normal width of these slope benches in valleys occupied by streams 

 of like volume. Left in its undisturbed state, it would have been classed 

 with the other like structures. The only peculiar feature is found in 

 the more than usual readiness with which the granitic and volcanic 

 rocks of the ridge decay. These rocks disintegrate at a considerably 

 greater rate than the average of the mountain-building formations of 

 the area, with the result that the Butte ridge has evidently lost in height 

 more rapidly than the other divides of this region. The only effect of 

 these local conditions has been to hasten the processes by Avhich the 

 valley was formed, so that the trough is somewhat broader than it would 

 otherwise have been. 



The plentiful openings which have been made in the talus slopes be- 

 neath Butte show that the bed rocks form a tolerably continuous incline 

 in which there are no conspicuous departures from a regularly inclined 

 surface to aline about 1,000 feet from the general position of Silver Bow 

 creek; there they turn rather suddenly downward. On the opposite side 

 of the stream toward the now bald granitic hill known as Timber butte the 

 few pits that have been sunk indicate a similar position of the bed rocks. 

 The gorge into which these slopes of the firm rocks decline apparently 

 has a width of one-half mile. Its depth is uncertain, but as it is clearly 

 at least 400 feet deep in the small branch of the creek that lies to the 

 east of the town between it and the Continental divide, we may assume 

 a considerably greater depth of the buried gorge on the line of this cross- 

 section above mentioned. Assuming that this buried canyon has the 

 slope of the walls continued downward in the form of a V and with the 

 steepness they have near their margin, it is likely that the bed of the 

 ancient stream is at least 600 feet below, the present course of its waters. 



A careful inspection of the Silver Bow valley from the canyon on the 

 west to the head of the streams that drain it warrants the supposition 

 that the same form of the bed rocks exists in all its parts. Everywhere 

 the numerous prospect holes made in the search for ore-bearing veins 

 reveal the presence of the crystallines beneath from 5 to 50 feet of sur- 

 face debris out to a line from 1 to 2 miles beyond the mountain steeps, 

 and then a swift descent of the under rocks to a narrow and deep chan- 

 nel. As to the nature of the filling of this deep gorge we have no cer- 

 tain information. It is said, however, that wells near the stream have 

 encountered a whitish clay at the depth of 30 feet which may be of vol- 

 canic origin. Assuming that the valley of the Silver Bow has been thus 

 deeply excavated and refilled, the question arises whether this cavity 



XL— BuT,L. Geol. Soc. Am., Vol. 12, 1900 



