DIKES NEAR SYLVAN PASS. 307 



an intrusion of homblende-mica-andesite (1545), which is dense and dark 

 gray, with abundant hexagonal plates of biotite, numerous feldspars, and 

 inconspicuous altered hornblendes. 



Immediately north of the pass is an exposure of granite (153G, 1538). 

 It is massive and breaks into large blocks. The exposure is about 75 feet 

 high and 150 feet long. The rock is fine grained, with porphyritical 

 biotites, resembling those in the andesite just noticed, but less numerous. 



Dikes of light-colored andesite cut the high peak east of Sylvan Pass, 

 which is composed of dark-colored basic breccia. They have a general 

 trend a little south of east, and are found traversing the valley of Middle 

 Creek in the same direction. Here they cut the older basalts. At the 

 upper forks of this creek, a short distance east of Sylvan Pass, there is a 

 great dike of hornblende-mica-dacite (1546), with some quartz phenocrysts 

 and abundant large feldspars. The biotite forms comparatively long prisms, 

 one being 5 mm. in length. They taper slightly toward the ends. The horn- 

 blende is more or less decomposed. The dike is 80 feet wide, and forms a 

 high wall on the southern side of the valley, and passes across the south fork 

 of the stream. It is almost vertical, with slight hade to the north, and is 

 distinctly jointed in horizontal prisms. Parallel to this is a smaller dike of 

 compact hornblende-mica-andesite with multitudes of minute phenocrysts 

 (1547). The breccia in the immediate vicinity is indurated. Halfway 

 down Middle Creek on the north side are two dikes of dense gray rock 

 with very few phenocrysts of hornblende (1548, 1549). They have an 

 east-west trend. 



At Sylvan Pass, east of the divide, the valley is filled with masses of 

 rock from the dikes and breccia on either side. Some of the dike rocks are 

 dense crystalline andesite-porphyry, with abundant thin crystals of horn- 

 blende (1538, 1539); others are less crystallized, with various habits. On 

 the south side of the ])ass indurated breccia, cut by many parallel dikes of 

 andesite, forms almost vertical cliffs 500 feet high. Some of these dikes are 

 dark colored, dense, and lithoidal, with comparatively large hornblende 

 phenocrysts (1540); others are darker, with abundant, though not so promi- 

 nent, phenocrysts of hornblende (1541). Some are light gray and compact, 

 with extremely few phenocrysts, while some are altered and green, with 

 spots of epidote (1542, 1543). There is, however, not much alteration 

 noticeable in the vicinity, most of the rocks being fresh. The slopes 



