ANDKSITIC UOCKS XEAU SlLVl!:ilT()X, COLORADO 5 



become consolidated into breccia-like masses. No attempt was made to trace tbe 

 ditrerent Hows from one place to another, and tbe specimens were simj)ly taken at 

 random in the ditlerent localities. However, in vieAV of the fact that most of 

 them were collected at altitudes of from 12,000 to 14,000 feet, it Heems very prob- 

 able that the yoiin«rer flows were trathered. Most s])ecimens w^ere collected on 

 Kint; Solomon, Galena, and Boulder mountains, which lie in a northerly direc- 

 tion from Silverton, coverintr a triangular area. However, while there I had 

 neither maj) nor instrument with me and my points of the compass may not l^e 

 ver.v accurate. 



Tbe rocks of this locality, although apparently all of the same kind, differ con- 

 siderably in appearance and texture. The s])ecimen which was least altered, and 

 therefore most typical, was found at the summit of the southeast spur of C-Jalena 

 mountain, above tbe Big Ten claim, which at that time was being exploited by 

 Mr A. M. Campbell and associates. It is this rock which will serve as the basis 

 of the description for all the rocks of the region. Nevertheless, various charac- 

 teristics not found in this specimen but observed on others will be incorporated 

 into the description of this rock, in order that a general account may be given 

 without taking too much time and space. 



Megascopic Description of the Rock 



Megascopically the rock in question is of gray color, with numerous phenocrysts 

 of plagioclase feldspar, hornblende, and magnetite in an aphanitic groundmass. 

 The phenocrysts are arranged in some instances in roughly parallel directions as 

 a result of flowing. This rock effervesces but slightly wntb acids, while others do 

 so very strongly. The prevailing color of the rocks is grayish green, but at times 

 a brownish to reddish gray was noted. Sometimes the phenocrysts are very 

 numerous and give tbe rock almost a plutonic appearance, while again they are 

 nearly absent, and the mass has an aphanitic character. Often the plagioclase 

 crystals are from 3 to 5 millimeters long and 2 to 5 millimeters across. However, 

 in tbe rock from Dives tunnel, given in tbe analyses, the feldspars are at times 19 

 millimeters long and from 6 to 13 millimeters wide. When unaltered tire cleavage 

 faces have pearly luster, but they are quite commonly dull. The hornblende is 

 black, with smooth glistening cleavage planes, and is not found in very large 

 amounts. Magnetite is plentiful and is generally titaniferous, as is shown by mi- 

 croscopic examination. 



Microscopic Observations 



Microscopically, the rocks are found to be distinctly holocrystalline porphyritic, 

 although at times the groundmass is so intensely fine as to be almost cryptocrys- 

 talline. 



Plagioclase phenocrysts occur in various forms, but are probably generally tab- 

 ular parallel to the brachypinacoid (010), being bounded by the prisms (110) (110), 

 brachypinacoid (010), base (001), and a macrodome. The crystals are generally 

 twinned after the Albite law, and very frequently compound twins after Albite 

 and Karlsbad laws are found. Pericline twins occur more rarely. Zonal structure 

 is frequently observed, and at times is very common. When such structure is 

 noticed, the extinction angles of the center are always found to be considerably 

 greater than those of the periphery, which indicates isomorphous growths of the 



