104 GEOLOGY OF THE COMSTOCK LODE. 



the same changes as the hornblende and augite described under 194 and 

 281. As would naturally be supposed, the change to epidote begins along 

 cleavage lines. The change is illustrated in Fig. 8, Plate II. 



Slides 482, 485, 486. East-and-west dike, just soutli of Eldorado croppings. 



Dikcofdiorite-porphyry. — At thls point a dike of porphyritic diorite about six 

 feet wide cuts the granular mass of Mount Davidson. Towards the center 

 the rock is fine-grained but evidently crystalline, with small porphyritic 

 crystals of feldspar and hornblende. For about an inch from the edge the 

 rock is verj" dark and crypto-crystalline. The contact with the granular 

 diorite is an absolutely sharp mathematical line, and the adhesion is xovj 

 strong. Under the microscope the gray including rock is precisely such 

 as is described under slide 213. The adjacent dark rock is manifestly 

 the same as^'il. It is very andesitic in appearance, showing a microlitic 

 groundmass, with excellent flow structure, and solid brown hornblendes with- 

 out black borders. It also contains a few green fibrous hornblendes, and a 

 good deal of augite. Even within the limits of the slide, however, it is ap- 

 parent that the structure of the groundmass is more granular as the distance 

 from the contact increases. In slide 486, from the center of the dike, almost 

 the whole of the hornblende is fibrous, the structure is granular, and the 

 impression is simply that of an ordinary granular diorite with a few por- 

 phyritical crystals of feldspar. But a few of the hornblendes are partly 

 brown and solid, and these portions pass into and are surrounded by green 

 fibrous hornblende of the same crystallographic orientation. 



MICACEOUS DIOEITE-POKPHYRY. 



Slide 101. 1,000 feet northeast of Silver Hill mine. 



Typical example. — This is a gray-grccu porphyry, in which crystals of feld- 

 spar of a very uniform size, about half as large as a grain of wheat, and 

 smaller crystals of mica and hornblende, are evenly distributed in a crypto- 

 crystalline groundmass. Under the microscope apatite, titanic iron, and 

 zircon also make their appearance. 



