40 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
optical properties. Feldspar crystals were separated from specimens 
of the porphyry taken from different localities and their specific gravity 
was determined. The range was from 2.G to 2.62. On account of the 
presence m the heavier feldspars of minute inclusions of piedmontite, 
the lowest specific gravity was considered to represent the purest 
feldspar. 
Twinning is very common, according to either the Carlsbad or Mane- 
bacher (periclme) laws. PI. XV, b, shows a Manebacher penetration twin 
of feldspar. The section is nearly parallel to the clinopinacoid, as the 
following observations indicate: The axis of least elasticity makes an 
angle with the twinning line (rhombic section) of about 15 degrees on 
one side and 21 degrees on the other; the obtuse positive bisectrix 
emerges. There is a slight trace of a basal cleavage and a microper- 
thitic intergrowth parallel to 6. The position of the plane of the optic 
axes, the distribution of the axes of elasticity, and the specific gravity 
all indicate that the crystal is the triclinic soda feldspar anorthoclase. 
The resemblance to this of the other feldspar phenocrysts makes it 
probable that this is the prevailing feldspar. Other sections show the 
albitic intergrowth (microperthite) developed in a pronounced manner 
(PI. XVI, a). Every gradation of this structure is present, from the 
microperthitic to the cryptoperthitic. Sometimes the perthitic growth 
does not persist throughout the crystal, but is present in an incipient 
stage along its edge. 
The feldspars are often cracked and drawn apart in the direction of 
the schistosity of the rock, and the cracks cemented with sericite scales 
whose parallel arrangement conditions the schistosity (PI. XVI, b). 
This is most striking in porphyry obtained 10 feet below the surface, 
which shows an abrupt passage into a sericite-schist. The phenocrysts 
are crushed and pulled apart. This action had been accompanied by 
an abundant development of sericite. Some of the phenocrysts pos- 
sess in the hand specimen a red color, which is due to a fine admixture 
of red iron oxide. These phenocrysts frequently prove, from a test of 
their optical properties, to be orthoclase. 
The alteration of the feldspars to epidote may indicate the pres- 
ence of lime in the feldspar. Undoubtedly, however, much of the lime 
is of secondary origin. This alteration of feldspar to epidote seems 
to be a direct one. There is no intermediate kaolin stage, such as 
observed by Eutley. ! Accompanying the epidote is granular quartz. 
These two alteration products usually occupy the center of the crystal 
and are surrounded by a rim of unaltered feldspar. 
Brilliantly colored piedmontite frequently fills cavities in the feldspar 
crystals. This mineral is often surrounded by a rim of epidote. 
Quartz. — The bipyrainidal or rounded quartz phenocrysts are remark- 
able only for their undulatory extinction and the cracks by which strain 
• Rutley, On some perlitic felsites and on the possible origin of some epidosites: Quart. Jour. 
Geol. Soc. London, Vol. XLIV, 1888, pp. 741-742. 
