33G GEULOGY AND MINING INDUSTRY OF LEADVILLE. 



ofteu visible on macroscopic crystals. It occurs citbor as a uiacroscopic element of the 

 rock, in tbe form of minute needles in the groundmass, or, lastly, in clusters of small 

 irregular individuals, and then usually associated with biotite leaves. 



The small needles are sometimes well terminated (see Fig. 3, Plate XX). Still 

 it is tbe rule to find the ends irregular, while the prism is sharply defined (see Fig. 4, 

 Plate XX). Tbe pleocbroism is well marked, and tbe maximal angle of extinction in 

 the prismatic zone is nearly 18°, measured from the vertical axis. Twinning parallel 

 to ctj P av is common and is frequently polysyntbetic. 



Tbe hornblende occasionally includes crystals of apatite, magnetite, rarely biotite, 

 and clear microlites of zircon. It is commonly very fresh, and when decomposition 

 has begun the Hrst product is usually chlorite, from which epidote is formed. 



Quartz. — Tlieic are but few rocks of this kiud in which quartz is prominent as a 

 macroscopic constituent. In some of these, usually the more acid ones, it forms well- 

 defined crystals, but it is more common to see it in rounded grains, seemingly quite 

 variable in quantity, in occurrences which are otherwise nearly identical. These 

 rounded particles undoubtedly represent partially remelted crystals of the first gen- 

 eration, and their variability is here not remarkable. Tbe chief development of the 

 quartz is, as perfectly natural, in tbe groundmass, with orthoclase. Inclusions are not 

 abundant in any of the crystals, though all earlier minerals do penetrate it in observed 

 instances. Glass inclusions have never been found and those with fluid contents are 

 rare. The groundmass seldom penetrates tbe large crystals. 



Groundmass. — The groundmass of those porphyrites which contain hornblende 

 and biotite mainly as macroscopic elements is very uniform iu constitution and struct- 

 nre. It consists of an evenly granular mixture of quartz and feldspar, with small 

 octahedrons of magnetite scattered through it. Tbe feldspar is seldom definitely 

 determinable as such, but its pi-esence is inferred from a formation of muscovite, 

 where the rock is much altered, and because tbe quartz grains, through their stronger 

 polarization, stand out in contrast to tbe rest of tbe colorless groundmass. ■ By far the 

 greater part of this feldspar is monoclinic, for plagioclase was observed to enter into 

 the composition of the groundmass in but few cases, and then in tbe form of thin 

 plates, quite distinct from the irregular grains of orthoclase. The average size of 

 the grains of quartz and orthoclase is 0.02""", so that a complete separation of 

 these minerals is never possible. There is never a trace of microfelsitic or glassy sub- 

 stance, and only in contact specimens is tbe greater part of the groundmass cryiito- 

 crystalline. As has been mentioned above, biotite and hornblende enter into the con- 

 stitution of tbe groundmass in very varying quantities, and only when present iu 

 great abundance do they render the mosaic of quartz and feldspar obscure. The 

 quartz has a tendency to develop in clusters of irregular clear grains in certain cases. 



The distinguishing peculiarity of a certain minor subgroup lies chiefly in the 

 character of tbe groundmass. This consists principally of an intergrowth of quartz 

 and orthoclase, according to no discernible law, now the quartz, now tbe orthoclase 

 being the inclosing mineral, and their relation is only made clear between crossed 

 nicols, when it is seen that within the limits of certain irregular patches all tbe quartz 

 and all tlie orthoclase has each its own optical orientation. The outline of the inclos- 

 ing mineral has uo relation to crystal form, and this intergrowth actsthrougiiout like 

 the ordinary groundmass, tilling the interstices between the large crystals. The macro- 



