No. 2318. PETROGRAPHY OF DIKE ROCKS OF IDAHO— SHANNON. 487 



somewhat sericitized. Interstitial quartz grains which occur fre- 

 quently may be secondary. (See pi. 37A.) 



Lam.prophyre, Grouse Tunnel, Grouse Gulch. — This rock is megas- 

 copically one of the coarsest of the lamprophyres. It is of medium- 

 gray color, having feldspar in excess of ferromagnesian minerals, 

 and has prominent black spots which apparently are small aggre- 

 gates of hornblende. Under the microscope it appears as a com- 

 paratively coarse holocrystalline aggi-egate of feldspar with slightly 

 subordinate ferromagnesian minerals consisting of hornblende and 

 biotite in about equal amounts. The feldspar consists of both ortho- 

 clase and plagioclase, the former slightly in excess. 



Olivine-Kersantite, Spring Gulch. — Just west of the mouth of 

 Spring Gulch on the south side of the river near Osburn, two parallel 

 dikes are exposed a few feet apart, cutting rocks of the Prichard 

 formation. These dikes were examined by the writer, but in the 

 following description Mr. Calkins' specimens and sections were 

 utilized. Only the largest and freshest dike is represented. In the 

 hand specimen the rock is dark gray to black, fine grained and non- 

 poi*phyritic, showing under a lens small flakes of biotite and brown 

 spots which innj represent serpentinized olivines. Under the micro- 

 scope it shows the usual trachytic aggregate of plagioclase laths with 

 predominant biotite and accessory hornblende as the ferromagnesian 

 minerals. The plagioclase is in well-defined laths, characteristically 

 zoned and twinned. Sericitization is well advanced in all cases, be- 

 ginning with the more basic cores of the crystals and progressing 

 outward. Augite is common in disseminated ragged crystals and 

 large pseudomorphic areas of both talc and serpentine, dotted with 

 iron ore, which are in this case surrounded by a rim of biotite, 

 clearly represent original olivine. The usual accessory apatite and 

 iron ore are present. 



Minettes, other occurrences. — Other much decomposed biotite-lam- 

 prophyre dikes occur in the district, especially near the Enterprise 

 Prospect, near the mouth of Polaris Gulch, and near Raven, on 

 Prichard Creek. A typical fresh rock of this type is exposed in the 

 workings of the Moonlight claim above Burke, but it was not studied 

 in thin section. 



II. HORNBLENDE TYPE. 



Odinite, Ilecla 3fine. — Ransome ^ describes the dike which accom- 

 panies the Hecla Vein as occupying the same fissure as the lode and 

 having an average width of two feet but in places reaching 7 feet. It 

 is not quite simple, but branches and pinches out to be succeeded by 

 another dike a few feet to one side. The dike is clearly younger than 

 the ores and sends out minute branches which occupy little fissures 



'Prof. Paper, D. S. Geol. Survey. No. 62, p. 176. 



