GRAY PORPHYRY. 331 



The Gray Porphyry is seldom fresh, as it occurs in the region adjacent to the 

 ore deposits, where agencies of alteration have been active, and presents usually a 

 greenish-gray rock, showing numerous crystals imbedded in a prominent grounduiass. 

 The minerals are the same as those of the Lincoln Porphyry, viz. large orthoclase, 

 small and numerous plagioclase, and biotite crystals. In the mines the rock is so 

 bleached that even jvith its original large crystals, it is not easily distinguished from 

 the White Porphyry. The quartz contains large bays or penetrating arms of the 

 grouudmass. 



Microscopical. One never-failing and striking peculiarity of this, in distinction 

 to the Lincoln type, is the presence of outlines of a former constituent of the rock, which 

 would seem to belong to hornblende, although no trace of that mineral in fresh condi 

 tion could be found. These outlines are usually marked by dark grains, and inclose 

 a fine grained, grayish decomposition product, which acts very feebly in polarized 

 light. They are not wanting in any slide examined, and are always of the same 

 appearance, even when other minerals are entirely fresh. 



The feldspars of the Gray Porphyry, unlike those of the Lincoln Porphyry, con- 

 tain numerous fluid inclusions, which are generally arranged parallel to the chief cleav- 

 age planes. Besides these, there are many irregular interpositions, either devitrifled 

 glass inclusions or portions of the groundmass in a less crystalline state than it now 

 presents in the main mass of the rock, They are light reddish-brown in color, and plen- 

 tiful in most of the small crystals. Distinct glass inclusions, although not noticed in 

 any feldspars, are very characteristic of the quartz grains. They are often sharply 

 negative crystalline in form, and sometimes show devitrification; others are spherical, 

 and in these it can often be seen that from opposite poles, which probably lie in the 

 vertical axis of the quartz grains, cracks penetrate the sphere in three planes, cutting 

 each other at about 60. If the sphere be cut by the section at right angles to the 

 axis uniting these poles and near one of them, there results a delicate six-armed figure, 

 which appears as if contained in the quartz itself. The groundmass, though holocrys- 

 talline, is much finer-grained than that of the Lincoln Porphyry, and shows a tendency 

 to an irregular intergrowth of quartz and feldspar. 



Occurrence. Gray Porphyry is quite limited in distribution, being confined to 

 the immediate vicinity of Leadville, and to the region northwest of that point. As 

 .has been described in detail (p. 80), it occurs chiefly in one large sheet, with numer- 

 ous offshoots, and the large sheet has been directly traced to a connection with an 

 immense body at the headwaters of the Eagle River. The hornblende of the Gray 

 Porphyry is considered analogous to the crystals of that mineral observed in small 

 dikes which are offshoots from the Eagle River mass. 



Chemical composition of the Lincoln and Gray Porphyries The following rock analyses 



were made by W. F. Hillebrand. 



I is of Lincoln Porphyry, summit of Mount Lincoln [75]. It is quite fresh in 

 appearance, although showing some muscovite, calcite, and chlorite 3 when examined 

 microscopically. 



