76 GEOLOGY OF THE COMSTOCK LODE. 



instance, though the paragenesis of the products of decomposition was a 

 subject of special inquiry. On the other liand, the formation of epidote at 

 the expense of chlorite is proved beyond a doubt. The development of 

 epidote usually begins near the centers of patches of chlorite, sending out 

 faggot-like masses of crystals in all directions, and ultimately, under cer- 

 tain conditions, occupying the whole space. In certain stages this process 

 can be admirably observed, long prismatic needles of epidote extending into 

 the chlorite and cutting the minute tibers of the latter at all sorts of angles. 

 This is peculiarly well seen in Figs. C and 7, Plate II. Sometimes the 

 chlorite-fibers, at the periphery of hornblende pseudomorphs, exhibit a spec- 

 ial arrangement, lying strictly parallel to one another and perpendicularly 

 to the crystal face. These fibers are often nearly of the same length, and 

 thus form a belt or zone. Such a belt must be denser than a spherolitic 

 mass, and not infrequently appears to offer a greater resistance to the forma- 

 tion of epidote. This is in accordance with the chemical conditions, for the 

 transformation cannot take place without the access of solutions. Complete 

 pseudomorphs of epidote after hornblende may result from this process undei" 

 favorable conditions, and such an one occurring in a slide from the McKihJx'u 

 Tumid is shown in Fig. 9, Plate II. From a study of a series of slides from 

 the same locality it is evident that this pseudomorph is the last stage of the 

 process illustrated by Figs. 6 and 7, Plate II., and not a case of direct con- 

 version. Epidote is also constantly found developing in patches of chlorite, 

 which occur in the groundmass of the rocks, where they have ap2:)arentlv 

 been depo.sited but not formed; and microscopic \eins of chlorite are 

 common in which various proportions of the mass are changed to epidote. 



Feldspar does not decompose to epidote. WlieU the fcldsparS beCOmC pOrOUS, aS 



they do so soon as decomposition has commenced, they are subject to infil- 

 tration by chlorite, and the chlorite so deposited is converted into epidote 

 under the same conditions as in other portions of the rock. One distin- 

 guished lithologist has attributed the supposed, but confessedly mysterious, 

 alteration of feldspar into epidote, to the presence of plentiful hornblende- 

 needles embedded in the feldspar. In the section of this chapter dealing 

 with propylite it is shown that this determination is erroneous, the supposed 

 hornblende particles in the slide upon which the suggestion is founded being 



