700 IDAHO MINING DISTRICTS. 



GEOLOGY. 



The prevailing rock is the normal gray granite of the Boise Moun- 

 tains, composed of orthoclase, plagioclase, quartz, and biotite ; horn- 

 blende is of rare occurrence in it. A jointed structure or sheeting is 

 often noted, the direction (N. 50 to 80 E.) and dip (up to 45 S.) 

 roughly corresponding to those of the veins. Numerous dikes cut the 

 granite, and may be divided into several classes. 



Some of the dikes consist of a harder gray granite, which carries 

 some muscovite. Less frequent are dikes of coarser and more mica- 

 ceous character than the general mass. Along the north side of Wood 

 Creek are several dikes of a pyritiferous granite, occurring at one 

 place as foot wall to a vein. 



The most prominent dikes are those which crop so boldly on Black 

 Creek, about 3 miles southwest of the Homestake mine, and which, 

 according to Mr. Howe, continue for a long distance northward with 

 a general trend of N. 5 W., one of them, the most easterly, showing 

 on the map Their width is up to 200 feet. The rock, which has a 

 somewhat porous character, is dark gray in color with brownish spots; 

 phenocrysts of feldspar are abundant, and are usually about 5 mm long. 

 It is almost impossible to obtain fresh rock. The microscope shows 

 the rock to consist of sanidine phenocrysts; small, brownish, decom- 

 posed foils and prisms, probably decomposed biotite; and a holocrys- 

 talline groundmass, of spherulitic and micropegmatitic character, of 

 orthoclase and some quartz. The rock should probably be classed as 

 a syenite-porphyry. 



Normal granite-porphyry is common and forms dikes, more rarely 

 irregular masses, with a general direction of N". 20 to 30 W. , several 

 occurring on the divide between Wood and Grouse creeks. 



Lastly, there are narrow dikes of lamprophyres, dark-green, dense 

 rock, in which small foils of black mica are often seen. These vary 

 from 18 to 30 inches in width and often trend with the vein N. 78 E. 



A specimen of thislamprophyric dike rock from the Hidden Treasure 

 mine is a panidiomorphic granular rock composed of brown horn- 

 blende, augite, and soda-lime feldspar in slender, lath-like forms; 

 probably also some orthoclase. The rock is very similar to certain 

 camptonites, or, perhaps, stands between a minette and a camptonite. 

 Similar dikes also occur in the Homestake mine, and in the foot wall 

 of the High Five is a dike 15 feet thick, of lamprophyric rock with 

 abundant black mica and porphyritic orthoclase crystals. These 

 dikes are sometimes the hanging wall of the veins, at times appar- 

 ently not affected by the vein processes, at others partially or entirely 

 replaced by ore. At one place in the Homestake mine a part of this 

 dike, crushed and altered, lies in the middle of a 4-foot vein. Mr. 

 Howe concludes from his observations that the lamprophyres are the 

 oldest dikes, followed by the large dikes of syenite-porphyry, and 



