30 GEOLOGY OF TDK COMSTOGK LODE. 



taken into consideration. In certain localities underground, the granular 

 diorites contain much deep-brown and solid hornblende, and the speci- 

 mens which show this variety are manifestly fresher than those from 

 tlie localities where green hornblende occurs exclusively. In some cases 

 an alteration of the brown to the green variety is strongly suggested, while 

 in one series of porphyritic diorites it can be actually proved. It is therefore 

 altogether probable that the surface diorite originally contained some brown 

 hornblende, which has been changed to the green, fibrous modification by a 

 process analagous to the formation of uralite. To what extent the fibrous 

 hornblende has been derived from the brown mineral, there is at present no 

 means of inferring. 



Augite. — Augite is comparatively rare in the unquestionable granular 

 diorites, though I have observed it in a few instances. It is much more 

 common in the porphyritic diorites, and it may be that its absence from the 

 granitoid variety is due to conversion into uralite; for since determinable 

 crystal sections are seldom met with in this rock, it would be impossible to 

 distinguish secondary from primary green fibrous hornblende. Close to the 

 McKibhen Tunnel angular fragments of what appeared macroscopically to be 

 the dark fine-grained diorite frequently encountered in the district, and 

 especially well developed in this tunnel, were found embedded in light- 

 colored granular diorite. Under the microscope the inclosing mass exhibits 

 no peculiarity; but the inclosed rock, unlike the similar occurrences in the 

 same locality, shows abundant augite and almost no hornblende, though 

 structurally resembling the dark diorite. As the distinct diabase eruptions 

 are manifestly later than those of diorite, I am wholly at a loss for an expla- 

 nation of this case, except on the supposition that it represents a local and 

 exceptional substitution of augite for hornblende. This hypothesis, how- 

 ever, is so contrary to ordinary experience as to be exceedingly objection- 

 able, though were it true it would also serve to explain the ill-defined patch 

 of diabasitic rock in Ophir ravine, which is, like that just mentioned, much 

 more granitoid than the mine diabases, and has no apparent structui'al con- 

 nection with them. There can be little doubt that local modifications of 

 massive rocks in which the mineralogical composition is characteristic of a 

 distinct but allied rock-species, have been met with in various localities in 



