holmes.] VALLEY OF ROCK CREEK. 63 



between our station and the main summit, and joining the main stream 

 below. In the upper course, therefore, it is in granite, in the middle 

 part, in Paleozoic rocks, afterward in granite, and finally again in the 

 stratified rocks, cutting its way, after joining the main stream, from the 

 Silurian out into the Upper Cretaceous. Rock Creek proper, the 

 sources of which were explored last year, can be traced far to the south- 

 ward, as it comes down through a deep valley. This valley widens as 

 it approaches our station, but on reaching the belt of granite, suddenly 

 closes into a deep and precipitous canon. This is immediately under 

 us, to the west, and a stone dislodged plunges down over the crags 

 to the creek-bed, 1,500 feet below. This canon is hardly more than 

 half a mile in length (the width of the granite arm) and opens below 

 into the great triangular valley about the creek junction. 



The sketch on the opposite page, Figure 4, will aid in making clear the 

 geology north of our station, and about Sopris. As seen in the drawing, 

 there is considerable irregularity in the disposition of strata. The isolated 

 fragment of Paleozoic beds between Sopris and Eock Creek evinces a 

 considerable amount of lateral crushing as indicated by a series of 

 abrupt and angular concentric folds or wrinkles. Along the line of the 

 creek-bed there has probably been a slight dislocation or fault as indi- 

 cated by the want of harmony between the strata at e" on the east side, 

 and those at el" on the western. There are also traces of a considera- 

 ble degree of metamorphism, shown by the change of color near the 

 granite contact, as well as by the thinning-out of the entire series, such 

 as would occur in a number of sheets of iron heated and partially welded 

 together at the edges under the irregular blows of a hammer. It has 

 been suggested that the existence of a shore-line about the granitic area 

 has, during Paleozoic times, produced this thiuning-out, and especially 

 since, on the eastern face of the mountain, the more modern deposits 

 seem to jut up against or almost against the granite, but it should be 

 observed that in every locality where this peculiar granite appears, there 

 are unmistakable evidences of abrupt and violent movements, inde- 

 pendent of its relations to the sedimentary strata. It acts in all cases 

 as a foreign element, plastic and aggressive, intruding itself upon a 

 region heretofore undisturbed, and producing disturbances of the most 

 marked and unusual kind. In this case, however, there is less evidence 

 of violence than in any of the cases farther south, but in lithologic char- 

 acter and methods of upheaval it is the same. 



GEOLOGY OF THE DISTRICT DRAINED DY ROCK CREEK. 



We had resolved to follow the course of the western or main branch 

 of Eock Creek. In the canon there was an old trail leading over the 

 rocks, and we passed through, without difficulty, into the open valley 

 above. Here were some beautiful meadows in which we discovered a 

 group of hot springs. In a number of places, steam could be seen rising 

 from the grass and reeds, and on approaching we encountered a number 

 of slimy pools, from which considerable streams of hot water were flow- 

 ing. In all there were more than a dozen active springs, in most cases 

 impregnated with sulphur, and ranging, in temperature, from 30° to 

 104° V. 



On the 29th a rain-storm had set in, and everything was now wet, 

 thoroughly saturated. Muddy torrents poured down the upper slopes 

 and dashed over the cliffs into the valley. Avalanches of wet earth, 

 carrying many rocks and trees, formed near the summits and came roar- 

 ing down, discharging their great masses of debris into the river, and 



