56 GEOLOGY OF THE COMSTOCK LODE. 



of extinction exceeding those of labradorite, only a few such sections gave 

 above 32°, while many of the remainder gave within a degree or two of 

 31°. But I know of no way of absolutely proving this point. The feld- 

 spars very often show a zonal structure. A beautiful case of this kind is 

 mentioned under slide 20. Simply twinned feldspars are rare, and most 

 are polysynthetic, according to the albite law ; pericline twinning is very 

 common, and both of these sometimes appear in combination with Carlsbad 

 twinning. The stripes are ordinarily fairly uniform, and of considerable 

 width; but sometimes one or both sets are exceeding!}- fine, and not uncom- 

 monly they do not penetrate the crystal, so that one end shows stripes while 

 the other does not. It need scarcely be said that in such cases the unstriped 

 portion if ffivorably placed may be proved to be triclinic by its optical 

 properties. The porphyritical feldspars are usually developed in long lath- 

 like forms. The feldspars contain inclusions of glass in almost every slide, 

 either as negative crystals or as rounded bodies, and these, when fresh, ordi- 

 narily carry bubbles. Inclusions of groundmass too are common, and 

 inclosed microlites occur both of apatite and of what appears to be augite. 

 The latter are not sharply crystallized, and are generally fresh, though occa- 

 sionally accompanied by chlorite. They are light yellow, and sometimes 

 give angles of extinction of above 30°. I have seen no fluid inclusions 

 in such feldspars as seemed to be unaffected by decomposition. 



Other minerals. — The apatitcs aro usually colorless, but sometimes brown 

 and dusty. They seem to be universally distributed. Zircon occurs in only 

 one or two slides. The iron ore is for the most part magnetite, but occa- 

 sionally ilmenite is present. Fig. 19, Plate III., shows an excellent ilmenite 

 section from the highly augitic andesite in Cedar Hill Caiion, and the 

 application of chlorhydric acid established its presence with certainty in the 

 typical hornblende-andesite from near the ComUmilon shaft. 



The groundmass consists of feldspar microlites usually referable to 

 oligoclase, magnetite, and sometimes microlites of augite Fluldal structure 

 is conmion. Of course the groundmass nuist have crystallized in cooling, 

 and the question is suggested why the glass inclusions were not devitrilied 

 at the same time; but it is evident that a large part of each porphyritical 

 crystal must have formed after the glass was inclosed, leaving a residual 



