140 GEOLOGY OF THE COMSTOOK LODE. 



from these two specimens and slides, all the diagnostic points appear to 

 me to indicate diabase rather than augite-andesite as the proper determina- 

 tion. 



Exploration of the Fortieth Parallel Slide No. 224, specimen No. 21,259. Foothills 

 north of Tuscarora, Cortez Eange. 



This is a green porphyry, with impellucid feldspars and brilliant horn- 

 blendes. I am almost inclined to doubt that this can be the slide described 

 in the "Microscopical petrography;" but the dark brown hornblendes tally 

 precisely with the figure and the description, the slide corresponds to the 

 specimen, as does the latter with the locality, and no other slide labeled 

 "propylite" bears any considerable resemblance to the text and the illustra- 

 tion. The slide, contains a large number of unusually symmetrical brown 

 hornblende sections, with broad black borders. One or two of these exhibit 

 clinopinacoidal cleavage as well as the usual prismatic one. Many of the 

 hornblendes are altered into chlorite, still retaining the black border and 

 crystal outlines. This chlorite shows the usual aggregate polarization in 

 some places and spherolitic structure in others, and extinguishes light par- 

 allel to the direction of the principal Nicol sections. There are also many 

 fresh augites with characteristic sections, cleavages, and optical properties, 

 and pseudomorphs of chlorite after augite. As usual in decomposed rocks, 

 the groundmass contains irregular patches of chlorite, the properties of 

 which are identical with those of that in pseudomorphic forms. I could 

 find nothing whatever corresponding to the green hornblendes described by 

 Professor Zirkel, and figured as without black borders, and as showing 

 hornblendic cleavages and outlines. The feldspars and groundmass are like 

 those usually found in partially decomposed hornblende-andesite, and as 

 such T have no hesitation in regai'ding the rock. 



Exploration of the Fortieth Parallel. Slide No. 225, specimen No. 21,314. Wagon 

 Catiou, Cortez Range. 



This is a reddish rock, with well-developed porphyritic, impellucid 

 feldspars, visible mica, and greenish black patches, which are possibly 

 hornblendes. Under the microscope the rock is seen to be greatly decom- 

 posed. The slide contains fresh mica, numerous pseudomorphs of chlorite 



