OLDER ERUPTIYES. 



QUARTZ-PORPHYRY. 

 MOUNT ZION PORPHYRY. 



This rock occurs in the masses of Mount Zion and Prospect Mountain and is des- 

 ignated by a special coloring upon the detailed Leadville map, while it is united with 

 the White Porphyry on the map of the Mosquito Range. 



In structure it resembles a fine-grained granite at first glance, there being but few 

 biotite leaves, with occasional feldspar and quartz crystals, which by reaching a diam- 

 eter of three or four millimeters become conspicuous in the mass of the rock. When 

 the rock is fresh the naked eye easily distinguishes many quite uniformly small quartz 

 grains imbedded in the feldspar, which is the chief constituent. Biotite is uniformly 

 but sparingly present in small, irregular leaves. 



Microscopical By the aid of the microscope the following constituents are found, 



named in order of their formation: Zircon, 1 magnetite, apatite, biotite, plagioclase, 

 orthoclase, and quartz. 



With a low power of the microscope the chief part of the rock is found to consist 

 of an irregular granular mixture of orthoclase and quartz, the latter occurring in 

 roughly rounded grains 0.3 mm to 0.7 mm in size, which often seem inclosed in the more 

 irregular and frequently larger grains of orthoclase. The presence, in almost every 

 grain of these two minerals, of plagioclase inicrolites having a prismatic habit with 

 apparently somewhat rounded terminations, and averaging O.l mm in length by 0.01 mm 

 to 0.03 mm in width, shows their coincident formation. These microlites, which con- 

 sist of from two to five laminae, are very numerous and form the most character- 

 istic constituent of the rock. Plagioclase grains occur, corresponding in size to those 

 of orthoclase and quartz, but they usually show some crystal outlines, and through 

 their freedom from the microlites the correspondence of these grains to the larger, 

 stout crystals, which are sometimes 4 millimeters in diameter, seems clearly estab- 

 lished. Tbe total absence of the microlites, the difference in form, and the larger 

 angle of extinction, reaching in some observed cases 20 either side of the twinning 

 plane, show plainly that these crystals represent an earlier and doubtless more basic 

 variety of plagioclase thau the microlites. The larger crystals are not abundant, and 

 are seldom prominent in the hand specimen. Biotite is never developed in crystal 



1 In nearly all the rocks of this district a mineral, presumably zircon, has beeu found. Its identity 

 has been proven iu a rock from the Ten-Mile district, chemically and crystallographically. 



323 



