No. 2318. PETROGRAPHY OF DIKE ROCKS OF IDAHO— SHANNON. 481 



in Mr. Umpleby's collection also fail to agree with Mr. Kansome's 

 description as noted below, the only other rock known from the 

 district Avhich does agree with Mr. Ransome's specimen being the 

 hornblende-quartz rock from Bailey's Pond. 



In appearance this rock is essentially like that from the adjoin- 

 ing Marsh mine, being composed of biotite phenocrysts rather 

 sparsely scattered through a rather light gi'anular-looking gray 

 groundmass. 



Under the microscope this rock is seen to be composed of large 

 phenocrysts of biotite with accessory iron ore and well-crystallized 

 apatite with occasional 

 much altered pheno- 

 crysts of diopside in a 

 groundmass composed 

 chiefly of alkalic feld- 

 spar. Among the sec- 

 ondary minerals may 

 be mentioned abundant 

 sericite formed at the 

 expense of the feldspar, 

 infiltrated calcite, and 

 chlorite and secondary 

 amphibole formed from 

 the diopside. The bio- 

 tite as usual in these 

 rocks is intensely pleo- 

 chroic in tones of dark 

 and light brown with 

 dark reaction rims and 

 embayed outlines. It 

 occurs in well-formed 

 hexagonal tablets which 

 are markedly parallel 

 in arrangement and are 

 fresh and unaltered. 



Fig. 1.— DiAuitAM .suowing rel.vtion of dike to vein, Hecla 

 Mine. A. Dike. B. Crushed material with quaktz and 

 ORE. C. Galena. D. Quartzite wall rock. Width of 



section 15 FEET. 



The iron ore occurs in small scattered grains of euhedral outlines. 

 The apatite, which is abundant and rather late in crystallization, 

 forms well-crystallized stout hexagonal prisms. The groundmass 

 appears to be, in large part, composed of orthoclase, which occurs in 

 lathlike bundles of prismatic fibers which give a rather rolling ex- 

 tinction. This habit of the orthoclase gives the rock a somewhat 

 trachytic texture. Between the orthoclase laths are interstitial areas 

 filled with a material of very feeble birefringence which may be a 

 zeolite or perhaps glass. Scattered throughout the groundmass are 

 innumerable minute needles of granular appearance, high relief, high 

 144382— 20— Proc.N.M.vol.57 -31 



