74 GEOLOGY OF THE COMSTOGK LODE. 



degeneration of each of the various feldspar species has taken the same 

 course. 



Hornblende. — Hombleude in the diorites is met with, both brown and green 

 The brown variety is usually quite fresh, while the green exhibits a tendency 

 to a general degeneration throughout its whole mass. In one instance, at 

 least, it has been shown that the brown solid hornblende of a semi-porphy- 

 ritic diorite is altered into the green fibrous modification, and in other cases 

 tliere is strong reason to suspect a like change. Similarly, it has been shown 

 that the hornblende of the metamorphic diorite was in all probability once 

 colorless, and that it is now in part converted into a green modification of 

 a fibrous texture. The result in both cases is very similar to uralite. It is 

 by no means asserted that all the green fibrous hornblende of the diorites 

 in Washoe is an alteration-product of other varieties, though this seems 

 possible, but there is evidence enough to warrant calling the attention of 

 lithologists to the question how far green fibrous hornblende is to be con- 

 sidered the original form of the mineral. Professor Rosenbusch mentions 

 this change in connection with the proterobases of Lusatia. In the 

 younger rocks I have not succeeded in detecting a similar change. The 

 hornblendes of the Washoe andesites are either full brown, reddish brown, 

 or greenish brown in color. The tint of tliose last mentioned it is somewhat 

 difficult to describe, and consultation has shown that the definition proposed 

 depends considerably on the susceptibility of different eyes. To some they 

 appear green with a tinge of brown, while to otliers the green admixture 

 is scarcely perceptible ; but all agree that the color is very different from 

 the grass-green or bluish-green of the fibrous diorite hornblendes. 



Alteration of hornblende to chlorite. — Thc fibrous dioHtic horubleude, souic of the 

 brown variety in the porphyritic diorites, and all the hornblendes of the 

 younger rocks, appear to pass directly into chlorite. The attack seems to 

 take place from external surfaces and cracks. If the cleavages of the crystal 

 are well opened, each cleavage prism is attacked, and the result in longitu- 

 dinal section is a quasi-fibrated mass of rods of hornblende separated by 

 chlorite, and in cross-section a group of isolated rhombs or irregular patches 

 of the unaltered mineral embedded in chlorite, which often retains the outlines 

 of the original crystal in great perfection. Figs. 1, 2, and 3, Plate II., are 



