344 GEOLOGY AND MINING INDUSTRY OF LEADVILLE, 



last one [100-lOG]. In the hand specimen tbey show but little that can be ideutilied. 

 They are green in color and flue-grained, with some visible feldspar and biotite or 

 hornblende, and, rarely, quartz. The principal decomposition product is chlorite, which 

 renders the structure obscure. The microscope reveals a fully crystalline structure, 

 in which a granular groundmass of quartz and feldspar is of varying importance. 

 Quartz and orthoclase, intimately but irregularly intergrown, make up in some cases 

 the greater part of the rock. Muscovite is the chief decomposition product of tiie 

 feldspars and seems also to result from the alteration of biotite, after several inter- 

 mediate stages. 



The Green Porphyry and the ones just mentioned are now thought to be more 

 probably porphyrites than quartz porphyries. 



