LI.VDGREN.] DIKES ASSOCIATED WITH THE GRANITE. 683 



The porphyry begins at the Mountain Chief and Belzazzar mines, 

 where it occurs as a wide belt extending across the vein and forming 

 a considerable part of the hill to the east of the mines. Large masses 

 of a similar porphyry occur in the lower part of Fall Creek, and it 

 continues eastward toward Quartzburg as a narrowing belt. The 

 rock in this area is a characteristic light-colored hornblende-porphy- 

 rite, consisting of white plagioclase in stout prisms up to l cm long, 

 and idiomorphic hornblende crystals up to 5 ram in length, embedded 

 in a fine-grained groundmass of feldspar, hornblende, and a little 

 quartz. 



At Quartzburg the porphyry belt is only a few hundred feet wide 

 and has undergone great thermal alteration, secondary minerals like 

 pyrite, muscovite, and calcite being abundant. On the Gold Hill 

 mine dump specimens of fairly fresh rocks were collected. Some of 

 them are very similar to the above-described rock. The abundant 

 large white feldspars are labradorite, according to extinction of 

 numerous Carlsbad twins. There is no hornblende left undecom- 

 posed, but there is some light-brown biotite in process of conversion 

 to chlorite. The groundmass is micropoikilitic, consisting of quartz 

 and unstriated feldspar. Another variety, greatly altered, of a yel- 

 lowish color and impregnated with pyrite, is characterized by large 

 corroded quartz crystals, up to l cm in diameter, and large porphy- 

 ritic labradorite, greatly altered by sericitization. The groundmass 

 is fine-granular, probably micropoikilitic, of quartz and feldspar, but 

 now greatly filled with sericite. 



East of Gold Hill the porphyry belt widens considerably. Quartz- 

 burg Hill is composed of quartz-diorite-porphyrite. This rock, similar 

 to that described from the Belzazzar mine, forms the largest part of 

 the area. It crosses Wolf Creek as a wide belt, and is here accom- 

 panied by dikes of more basic rocks. Two miles north of Placerville 

 a dike of norite-gabbro crosses the road and a dike of normal diabase 

 appears in the same vicinity. East of Wolf Creek the area widens 

 still more and reaches its maximum width. At Sweet's Claim, one- 

 half mile west of Grimes Pass, the porphyry is narrower. A specimen 

 from the Northern Star shaft shows the same idiomorphic large feld- 

 spar and hornblende crystals. The former are very fresh and consist 

 of labradorite ; the latter are sharply idiomorphic and partly chlori- 

 tized. The groundmass is microcrystalline allotriomorphic, being 

 made up of quartz and unstriated, clouded feldspar. 



This porphyry belt was not followed east of Grimes Pass, but it is 

 apparent that it attains great development among the high hills rising 

 beyond it. Clear Creek contains a great abundance of porphyiy 

 bowlders. They are practically the same quartz-diorite-porphyrite 

 which is described above. Quartz is evidently always present in the 

 groundmass, and sometimes also as porphyritic crystals. 



