204 GEOLOGICAL SURVEY OF THE TERRITORIES. 



strata of No. 4 are particularly well developed, and afibrd important in- 

 formation as to vertical succession. As a rule, the paste is of dark color, 

 a bluish to maroon, sometimes with a greenish tinge, while the feld- 

 spars contaiued in it are yellowish. Small crystalline fragments of 

 feldspar are frequent in the higher strata ot this peak, and it is they 

 that usually impart the greenish color to the rock. They receive their 

 color in turn from a small percentage of protoxide of iron. Near the 

 summit is a baud of whitish rock, about 200 feet in thickness, that ap- 

 pears like a matrix, without any segregated minerals. 



Bescendiug from Handle's Peak we pass over an outcrop of meta- 

 morphic granite, in the short but deep caiion leading down to the main 

 creek, and there again reach volcanic rocks. Above this point we ascend 

 rapidly until the pass is reached, leading over to the Animas Forks. 

 With that the region of ore-bearing rocks begins, and that will be treat- 

 ed of in the chapter upon the San Juan mines. 



One of the main tributaries of Lake Fork is Godwin's Creek, the junc- 

 tion of which is opposite station 11. This creek drains a very high and 

 interesting section of country. Single prominent peaks of considerable 

 altitude are coutaiued in it, and the remainder, partly grassy plateaus, 

 partly small ridges, is to a great extent above timber-line. Station 8 

 is located at au elevation of 12,959 feet, on trachyte No. 3, and from 

 there a good view of TJucompahgre peak, the highest mountain of the 

 region, was obtained. On the station mentioned, the volcanic strata or 

 flows dip oft' to the eastward, incliniug slightly to the north. The rock 

 composing the summit of the occupied point is very hard, has a dark 

 paste, but weathers brown. Crystals of sanidite occur throughout. Hy- 

 alite may be ibund in small cavities or tissures. 



Ascending farther up the creek, one of the most important localities 

 of our district was reached, that about o mileseast of station 10, at camp 

 23. While all other points thus far visited had failed to offer any ex- 

 planation as to tlie locality from which the enormous quantities of 

 volcanic material flowed, the point just mentioned afforded sufficient 

 evidence regarding that interesting question. Near camp 23, as has 

 been mentioned above, the only place was found where basalt occurred 

 in a comparatively low country (10,G79 feet), and it was there where 

 rhyolite was observed to cover it. From the creek upward, on the 

 north side, steep, dark-colored bluff^s rose, cut into many irregularly- 

 shaped iragments by erosion. Above them a light colored mass of 

 rocks appeared, showing separation into distinct narrow strata, inclin- 

 ing at an angle thatreached 00° to 70°. The former is basalt, the latter 

 rhyolite. Of compact microcrystalline texture, the basalt showed numer- 

 ous inclosures of epigene minerals, chalcedony, agate, amethyst, 

 stilbite, and calcite. The three former occurred in small, perfectly- 

 filled geodes, while the two latter were generally found in fissures and 

 cracks. Immediately upon this rock lay the rhyolite, in the stated 

 position. A white to grayish paste, very compact, contains numerous 

 small crystals of trabsparent quartz with double terminations. Crystals 

 of sanidite are dispersed throughout the entire mass, and small six- 

 sided ])risms of black mica occur sparingly. Near the junction of the 

 rhyolite with basalt, a curious interstratum was found, only 6" to 8" 

 in thickness. It is a brown, vitreous pitchstone, containing crystals of 

 quartz and sanidite but no mica. This stratum was only traced for a 

 short distance, owing to the precipitous character of the walls, but 

 probably extends for some distance, analogous to the occurrences at 

 other points. Farther up the valley rhyolitic hills appear, light-gray to 

 white in color. Ascending the basaltic wall for about 700 feet, the 



