384 Ball — P re-Cambrian Rocks of Georgetown, Col. 



Petrography. — The Silver Plume granite is a medium- 

 grained hypidiomorphic granular rock which by the increase 

 in size of the pinkish-white porphyritic feldspars (Carlsbad 

 twinned microcline) and the decrease in size of the encircling- 

 quartz feldspar and biotite individuals passes near the contact 

 with older rocks into granite-porphyry. Some of the smaller 

 dikes are wholly of the granite-porphyry facies. The granite 

 at some localities is notably rich in biotite and is then dark gray 

 in color mottled by the pink feldspars ; at other localities the 

 biotite content is normal and the rock is pinkish-gray. 



Under the microscope the essential constituents of the 

 granite in descending order of abundance are alkali feldspars, 

 quartz, biotite and oligoclase and oligoclase-albite. Original 

 muscovite is lacking in some slides, in others it is associated 

 with biotite, and in certain rare instances is the only mica 

 present. Muscovite also forms large plates which poikiliti- 

 cally enclose quartz globules ; these plates grade into sericitic 

 shreds secondary to orthoclase and are themselves evidently 

 secondary to feldspar. The order of the consolidation of the 

 minerals from the magma is normal, although the periods of 

 separation somewhat overlapped each other. Quartz is of 

 chief interest from the constant presence in it of thread-like 

 opaque inclusions ; these are less abundantly present in feld- 

 spar. Of the alkali feldspars, microcline, often with micro- 

 perthitic bands, predominates in some slides and is lacking in 

 others. While sometimes occurring in secondary veins in 

 orthoclase and oligoclase, it is in the main an original constitu- 

 ent. Among the accessories zircon and apatite are abundant, 

 magnetite rather common, and ilmenite, pyrite, titanite and 

 rutile rare. 



The phenocrystic feldspars near the contacts of the granite 

 mass are arranged in well defined planes which follow the 

 sinuosities of the contact. The rock has not been recrystal- 

 lized and the parallel orientation and the poorly defined 

 cleavage consequent thereto are original structures due to 

 movements in the magma prior to final solidification. Second- 

 ary gneissic structure has only been formed over small areas 

 subjected to unusual dynamic movement. 



Age. — The Silver Plume granite cuts all of the formations 

 previously described and with the exception of the pegmatite 

 and associated granite and granite-porphyry is the youngest 

 member of the pre-Cambrian formations. 



Pegmatite and Associated Granites and Granite- Porphyry. 



Distribution. — These, rocks, the youngest of the pre-Cam- 

 brian formations, nowhere form large areas, but are, on the 

 other hand, present in small bodies in almost every outcrop 

 in the quadrangle. 



