bascom.] QUARTZ-PORPHYRIES. 41 
has been relieved. They are fresh and show characteristic ernbayments 
and inclusions. Like the orthoclase, they are sometimes reddened by 
inclusions of hematite, or are more rarely given a rich wine-red color 
by inclusions of piedinoiitite. 
GROUNDMASS. 
The noncrystalline groundmass of the porphyries consists essentially 
of quartz and feldspar. These minerals form either a finely micro- 
granitic mosaic or a structure which becomes of considerable interest 
in connection with the question of the original character of the crys- 
tallization. 
In the first case there is no reason per se for supposing the crystal- 
lization to be other than primary. It is only as the crystallization 
is associated with secondary structures, or structures testifying to a 
primitive glassy condition, that the presumption favors its secondary 
character. When the crystallization is microgranitic and j)resumably 
primary, the rock is a genuine quartz-porphyry. 
The structure alluded to, which occasionally replaces the quartz-feld- 
spar mosaic in the porphyries, is the micropoikilitic, 1 where all the quartz 
of the groundmass has crystallized in irregular areas inclosing the feld- 
spar as lath-shaped microliths. These feldspar microliths are oriented 
quite independently of one another. This structure characterizes the 
aporhyolites, and the question of its primary or secondary character 
will be fully discussed in connection with these rocks. 
The alteration of the groundmass to sericite and the formation of a 
sericite schist will be discussed in connection with the acid slates. 
ACCESSORY CONSTITUENTS. 
Piedmontite (manganese epidote) is the most remarkable of the acces- 
sory constituents. It is not only disseminated in microscopic quan- 
tities through the deep-red porphyry, but it also occurs in macroscopic 
masses, as a radiating aggregate, filling veins and cavities in the red 
felsite or replacing spherulitic crystallization. 
Two localities in the Monterey district furnish a great abundance of 
the piedmontite: the southwest flank of Pine Mountain, 1 mile north- 
east of Monterey station, and the hillside south of the Clermont House, 
between the turnpike and Minie Branch. Microscopic sections of the 
aggregate show brilliantly colored needles of piedmontite intergrown 
with clear quartz. (PI. XXV, b.) This intergrowth with quartz is also 
marked in the hand specimen. The radiating needles have the appear- 
ance of being broken, stretched apart, and the spaces filled with sec 
ondary quartz. The only other locality where piedmontite was found 
in macroscopic quantities is in the Buchanan Valley, 2 miles north of 
'G. H. Williams, On the uso of the terms poikilitic and micropoikilitic iu petrography: Jour, of 
Geol., Vol. I (No. 2, Feh.-March, 1893), pp. 170-179. 
