No. 2318. PETROGRAPHY OF DIKE ROCKS OP IDAHO— SHANNON. 496 



to describe each one separately is impossible. The above descrip- 

 tions, it is believed, cover the main types. The present paper is 

 intended as a more or less purely petrographic contribution which 

 is prepared to pave the way for a discussion of certain evidence 

 regarding the problems of ore genesis and modes of vein forma- 

 tion which, it is hoped, may be published later. The portion of the 

 conclusions of the present work which applies to the problem of 

 vein formation may be summarized as follows : 



1. Lamprophyric dikes in such widespread occurrence substantiate 

 the conclusion that the district is underlain by a granitic batholith. 



2. The fact that complementary aplite dikes failed to reach the 

 position of the present erosion surface gives some evidence as to the 

 depth at which this mass must lie. 



3. The coincidence in position of dikes and veins indicates a com- 

 mon source and argues a paucity of planes of weakness at the time 

 of their formation. 



4. The dikes are later in all observed cases than the veins with 

 which they are associated, yet they are all more or less affected by a 

 hydrothermal alteration which is traceable to the dying stages of 

 vein- forming activity proving that the dikes and veins belong sub- 

 stantially to the same general period. 



EXPLANATION OF PLATES. 

 Plate 37. 



A. Augite-Minette, Murray Hill. Abundant phenocrysts of biotite with scattered 



larger augites in a groundmass of orthoclase. Ordinary light. Magnitied 

 30 diameters. 



B, Olivine-augite vogesite, Helena-Frisco mine. Shows areas of talc, pseudo- 



morphous after olivine dotted with iron ore and surrounded with reac- 

 tion rims of hornblende together with large crystals of augite in a ground- 

 mass of hornblende and orthoclase. Ordinary light. Magnified 30 diameters. 



Plate 38. 



A. Porphyritic biotite vogesite. Variant of Elk Creek dike. Large biotite 



plienocryst in a mass composed of hornblende and orthoclase. The large 

 clear areas showing well-defined cleavage are calcite of obscure origin. 

 Ordinary light. Magnified 30 diameters. 



B. Spessartite, Eldorado Claim. Very coarse aggregate of prismatic hornblende 



and feldspar. White patches are holes in section. Ordinary light. Mag- 

 nified 30 diameters. 



Plate 30. 



A. Spessartite, Reeder Gulch. Showing contact of coarse and fine grained ag- 



gregates of hornblende and feldspar with large area of radiated epidote. 

 Ordinary light. Magnified 30 diameters. 



B. Minette, Hecla Mine. Shows inclusion of quartz surrounded by a reaction 



rim in an aggregate of biotite and orthoclase. Ordinary light. Magnified 

 30 diameters. 



