yg^^Ej GEOLOGY LA PLATA MOUNTAIN. 241 



which are somewhat concealed in many places hy the timber. The 

 granites in the caiion are beautifully polished, and rounded even more 

 noticeably than in the caiion of La Plata Creek. The rock exposed in 

 these canons is granitic — a granitic porphyry of a light color, with crys- 

 tals of feldspar, (orthoclase,) rather large and conspicuous, in a fine- 

 grained matrix : on weathered surfaces the crystals are of a purplish 

 color. 



On La Plata Mountain Dr. Hayden found evidence of volcanic intru- 

 sion. At a point in the caiion below the junction of the North Fork there 

 are also evidences of a trachytic dike. Following up the creek that 

 rises immediately west of La Plata Mountain, and ascending the ridge 

 opposite, we find trachyte. The western slope of the' ridge is covered 

 with debris, which weathers of a bright red color. The exposures are 

 limited. We find an obsidian rock, which seems to rest on the granite : 

 above this there is a gray layer ; then a light- colored trachyte 5 and 

 the summit of the hill is capped with a quartzitic- looking layer, the 

 weathered surfaces of which are bright crimson : from a distance this 

 color is very noticeable. There is a similar spot at the head of a small 

 creek rising southeast of Grizzly Mountain. This was visited by Mr. 

 Taggart, who reported as follows : " In Eed Eock Pass we have first 

 the quartzitic layer, the debris of which is bright red, and from which the 

 pass takes its name. Above this, as we pass eastward along the ridge, 

 we meet with varieties of trachyte. Over the first point, lying appar- 

 ently on the surface, there is granite and mica schist; next trachyte, 

 and again mica schist, "having an easterly dip, and an inclination of 

 about 80°. Then we have more volcanic rock, followed by a coarse con- 

 giomeritic layer, somewhat like the quartzitic layer first mentioned. It 

 contains pebbles of quartz. This continues for some distance, and is 

 succeeded by a green schist, (chloritici) Above this there is trachyte 

 again. Finally, we meet with a fine-grained granitic rock which con- 

 tinues for quite a distance, alternating with a volcanic breccia, above 

 which we have an obsidian rock, which is porphyritic." At the head of 

 the other branch we find granites and schists, having a dip a little north 

 of east. The schists are dark and micaceous, with seams of quartz. The 

 bedding is quite distinct in places, inclining at an angle of 40° to 50°. 

 In the smoothed rocks at' the head of the canon, we have abundant evi- 

 dence of glacial action. 



On the north side of the creek, high up on the ridge separating it from 

 the one next to the north, on which Mr. Taggart reported, there is a 

 capping of trachyte in columns. The rock^is of a reddish color, and 

 at one point seems to have included a portion of a bed of gneiss, as 

 shown at &, in Fig. — , Plate XIII, a a, being the trachyte. Grizzly Peak, 

 which was visited and ascended by several members of the party, was 

 also found to be volcanic. 



The figure in Plate XIV represents a block of volcanic breccia from 

 this peak. I was unable to follow the line of the eruptive rock so as to 

 define it accurately. In going up the creek, about a mile and a half 

 above the mouth of the North Fork, we first meet with it crossing the 

 creek in a direction south of east. The western line has a direction of 

 about north 25° west, and south 25° east ; on the north side it curves 

 more to the westward, while I think that not very far southward the 

 western line joins the eastern. The North Fork probably Hows partly 

 along the line of juncture, between the volcanic on the west and the 

 gneiss on the east. The extent of the outflow will have to be deter- 

 mined by closer investigation in the future; I do not think that it ex- 

 10 a s 



