Ball — Pre-Cambrian Pocks of Georgetown, Col. 381 



in plagioclase. Apatite grains in quartz are sometimes deeply 

 embayed and resemble the familiar corroded quartz pheno- 

 crysts of rhyolite. 



Near the borders of the batholith a rude gneissoid structure 

 is developed which microscopic examination proves to be in 

 part original and due to flow orientation of the phenocrysts 

 and biotite in a common plane, and in part a secondary struc- 

 ture due to granulation and slight recrystallization. 



Age. — Field observation shows that the quartz-monzonite is 

 younger than the gneissoid granite and older than the Rosalie 

 granite. Its relation to the quartz-bearing diorite will be dis- 

 cussed under that formation. 



Quartz- Bearing Diorite. 



Distribution. — Quartz-bearing diorite occurs in small stocks 

 and dikes which are largely confined to the northern one-fourth 

 of the quadrangle and are particularly abundant south of 

 Idaho Springs and northeast of Georgetown. 



Petrography. — The quartz-bearing diorite is a medium to 

 coarse-grained, rather uneven, granular rock, composed of 

 grayish-white striated feldspar, greenish-black hornblende, and 

 a little quartz. Biotite plates, which reach a maximum diam- 

 eter of 2 inches, poikilitically enclose the other constituents at 

 some exposures. Epidote occurs in matted films on joint faces 

 and in rude pseudomorphs after hornblende and biotite in the 

 rock mass. 



Under the microscope the texture is allotriomorphic or with 

 the partial development of plagioclase individuals hypidio- 

 morphic granular. The order of crystallization of the minerals 

 is normal, except that biotite in some cases solidified simultane- 

 ously with orthoclase .and quartz. Plagioclase varies from 

 andesine to bytownite, a basic labradorite being the most com- 

 mon species. It is characterized, as are quartz and orthoclase 

 in a less degree, by a vast number of minute tabular and cir- 

 cular interpositions which are black and opaque or clove-brown 

 and translucent. Plagioclase is altered more or less com- 

 pletely to sericite, zoisite and quartz. Green hornblende, in 

 elongated grains, often twinned parallel to 100, is usually rid- 

 dled with blebs of the other constituents." Quartz, which with 

 orthoclase forms wedges between hornblende and plagioclase, 

 is micropegmatitically intergrown with each species of feld- 

 spar, biotite and hornblende. Diallage, filled with the opaque 

 inclusions characteristic of the species, and surrounded by 

 secondary cores of hornblende, is a rather rare constituent, 

 although its j)resence is not surprising in view of the basic 

 nature of the plagioclase. In the alteration to hornblende the 

 dark inclusions of the diallage disappear, their substance 



