52 GEOLOGY OF THE COMSTOCK LODE. 



and clay seams can be seen very perfectly in some of the mines, as well as 

 extensive disintegration of decomposed portions, and there can be little 

 doubt that the behavior under erosion would be analogous. The pistachio- 

 green so often seen in the diorites and hornblende-andesites is less common 

 in the decomposed diabases, simply because the prevalent secondary mineral 

 is not epidote but chlorite. The chlorite is sometimes peculiarly distrib- 

 uted in blackish, rounded spots on a lighter ground. 



Diagnostic points. — Diabaso is likely to be confounded with diorite chiefly 

 when the feldspars have lost their transparency. The best indication macro- 

 scopically is then the lath-like feldspars, which are rare in diorite. The 

 granular fracture, though it may be very flue-grained, is usually sutiicieut 

 to separate it from augite-andesite. Hornblendic diabases in some cases 

 greatly resemble hornblende-andesites, which are often rather granular; but 

 hornblende is not very common or widely distributed in the diabase, and if 

 one specimen arouses a doubt, another can generally be found near by 

 which will set it at rest. 



Younger diabase. — Tlie "blaclv dikc" is a fcaturc which has long been ob- 

 served on the CoMSTOCK. It extends horizontally more than a mile through 

 some of the most important mines, and occurs from near the surface to the 

 lowest levels reached. It lies upon the foot wall, and is nowhere more than 

 a few feet in thickness. When fresh it is of dark-blue color and a granular 

 texture, without the least tendency to a porphyi-itic structure. Surfaces 

 which have been exposed only a few hours turn to a smoky brown tint, 

 a peculiarity shared by no other rock in the district. 



Under the microscope it is seen to be composed of triclinic feldspar, 

 angite, and magnetite. The feldspars are mostly developed in lath-like 

 shapes, and are of very uniform size. They give angles of extinction cor- 

 responding to labradorite. The augites are of the usual color, but seldom 

 well developed, and to a large extent occupy the interstices between the 

 feldspars. The rock is singularly free from inclusions of liquid or glass; 

 indeed, none such have been made out with certainty. The brownish tint 

 seems to arise from a suff'usion of the minerals with bi'own oxide of iron, 

 and this sul)stance is very likely produced b}' the oxidation of some chlo- 

 ritic mineral, of which, however, little is visible imder the microscope. 



