WHITE POKPHYRY. 77 



angular crystals. The muscovite as an original constituent occurs in spar- 

 ingly distributed, dark, hexagonal plates, which were at first supposed to be 

 biotite; their true character was learned only when a specimen was found 

 containing enough of the crystals to be subjected to optical and chemical 

 tests. (See Appendix B, Table I, Analysis II.) A characteristic appear- 

 ance of the rock is the frequent occurrence of pearly-white leaflets of mus- 

 covite, often in star-like aggregations, resulting from the decomposition of 

 the feldspars. Orthoclase is the predominant feldspar. No biotite has ever 

 been detected in the White Porphyry; but, as the rock is always in a more 

 or less advanced stage of decomposition and as biotite occurs in the Mount 

 Zion Porphyry, which seems to pass into it, it may have been an original 

 constituent, though it is rather remarkable that no traces of it exist even 

 in the small dikes where the rock still retains a distinct porphyritic struct- 

 ure and has a fresh conchoidal fracture. By means of the microscope are 

 found zircon as a common and magnetite and apatite as rarer constituents 

 of this rock. No glassy matter is found, either in groundmass or in inclu- 

 sions. Chemical analysis shows an appreciable amount of BaO and PbO, 

 substances common in the ores, in its composition. 



Among the miners it is known also as "block porphyry," on account of 

 its tendency to split up into angular blocks, which are often stained interi- 

 orly in concentric rings by iron oxide; and also as "forest rock," from the 

 frequent deposition of dendritic markings of oxide of manganese on the 

 cleavage surfaces. 



Occurrence. The principal development of the White Porphyry is con- 

 fined to a zone about the width of the Leadville map, and running from the 

 western boundary of that map south of east, instead of due east as the map 

 itself does. In other words, its lines have the prevailing northwest and 

 southeast trend of other larger features of the region. Within this zone it 

 is developed on an enormous scale, and occurs mainly as an intrusive sheet 

 directly overlying the Blue Limestone and in contact with the principal ore 

 deposits. It is not, however, entirely confined to this horizon, but is also 

 found at both lower and higher horizons and can sometimes be observed 

 crossing a stratum, generally at a low angle, from one horizon to another, 

 thus splitting the sedimentary bed into two wedge-shaped portions. This 



