40 GEOLOGY OF THE COMSTOCK LODE. 



slide from Cednr Hill the curious inclusions which seem probably ilmenite 

 needles, already referred to, are developed in great perfection. Most 

 of the hornlilende substance is concentrated in the larger crystals, but 

 there are a few minute ones and some crystalline fragments interspersed 

 through the groundmass. The larger feldspars are fairly well developed, 

 but have not the sharply rectilinear outlines so common in the diabases and 

 tlie volcanic rocks, nor do they display any tendency to elongated lath-like 

 forms. They give the angles of extinction appropriate to labradorite; they 

 contain occasional fluid inclusions, of rounded forms, and of course no glass. 

 They are pierced Ijy numerous apatite needles. The smaller feldspars are 

 in part crippled grains, similar to those of the granitoid diorites, and in 

 part elongated microlites. The angles of extinction of these latter render 

 it probable that they are oligoclase. 



The iron ore seems to be exclusively magnetic' The apatite is in part 

 of the ordinary colorless variety, and in part brown and dusty. The inde- 

 terminable inclusions in the apatites are disposed very diiferently in different 

 individuals. An hexagonal brown core is sometimes surrounded by color- 

 less apatite, while in other cases this arrangement is reversed. Longitudinal 

 sections not infrequently show colorless ends, with a dusty middle portion. 

 Only a few zircons have been observed in this rock, in which respect it dif- 

 fers from the gianular diorites. The groundmass consists of small feldspars 

 and magnetite grains, and its general eff"ect is usually that of an excessively 

 fine-grained granitoid diorite. Occasionall}' the arrangement of the micro- 

 lites is such as to suggest fluidal structure. 



Decomposition. — The dccouipositiou of these rocks forms an exceedingly 

 interesting stud}'. It will be shown elsewhere in detail that the hornblendes 

 pass into chlorite, and this again into epidote, quartz, and calcite. The chlo- 

 rite evidently possesses a high degree of solubility, and soon diffuses itself 

 through the groundmass, and through the feldspars so far as these latter have 

 become porous from decomposition. The chlorite and epidote give the par- 

 tially decomposed rocks their characteristic greenish hue. 



Another change of great interest appears in a small dike of porphyritic 

 diorite cutting granular diorite close to the Eldorado croppings. No effect 

 whatever has been produced upon the inclosing granular diorite, but for 



' Excepting the acicular iuclusions referred to above. 



