bascom] APORHYOLITES. 45 
In the former case the twinning - is sometimes repeated, and furnishes 
another indication of the triclinic character of the feldspar. 
The feldspars are frequently bent or broken, causing the twinning 
striations to show curvature and faulting. Occasionally a phenocryst 
has been completely broken, the fragments pulled apart, and the space 
filled with a quartz-albite mosaic of much coarser grain than the ground- 
mass, and evidently of subsequent formation. (PL XVII, a.) A com- 
plete replacement of the feldspar crystal by a quartz mosaic sometimes 
occurs. 1 
The quartz phenocrysts have rounded outlines and characteristic 
embayments and inclusions; undulatory extinction is almost universal; 
frequently the quartzes are cracked, and sometimes completely granu- 
lated. Both the porphyritical constituents of the aporhyolites give 
rise, in connection with curving now lines, to the " augen " structure 
which has often been described. 2 
These flow lines leave clear, triangular spaces on one side of the 
phenocrysts, such as Futterer describes.' 
Other porphyritic constituents. — The only ferromagnesian silicate 
unmistakably present in the aporhyolites is biotite. This was found 
in sections representing three different areas of acid volcanics which lie 
along the southern limit of the Monterey district. The first area is a 
small one at the head of Minie Branch. The second area is larger and 
caps the mountain southeast of Raven Rock Mountain. The third 
area is directly to the east of this, on Friends Greek. The lavas of 
these three localities show a marked similarity, both in the hand speci- 
mens and in the thin sections. They are compact, dirty-gray rocks, 
weathering a yellowish-gray. Feldspar phenocrysts are somewhat 
sparsely and inconspicuously present. On a fresh surface biotite can 
be detected with the naked eye. Magnetite is present, and in one of 
two specimens from the mountain locality pyrite is abundant in minute 
crystals. 
All of the thin sections of these rocks are distinguished by finely 
striated feldspars. The carlsbad and x^ericline twinning are sometimes 
both present in a single crystal. Quartz phenocrysts are not numerous. 
The presence of titanic iron oxide in abundance is attested by the 
formation of leucoxene. The groundmass is holocrystalline and finely 
microgranitic, with a tendency toward the micropoikilitic structure. 
This tendency is fully developed in one of the sections. The hand srjeci- 
men, from which this section was cut, shows a rock more completely 
silicified than the other specimens indicate, and also contains pyrite, 
which is indicative of secondary crystallization. 
•Similar replacements have been described by Dr. Milch, Beitnige zur Kenntnis des Verruca no, 
1892, p. 126, 
S J. Lehmann, Untersuehungcn uber die Entstehung deraltkrystallinischen Schiefergeateine, Bonn, 
1884. G. II. Williams, Bull U. S. Geoi. Survey No. G2, pp. 85, 118^ 207, PI. XV, fig. 1. 
3 Karl Futterer, Der " Ganggranit von Grosaaohsen und dor Quartz-porphyr von Thai nn Thiiriuger 
Wald, Iuaug. disser., 1890, p. 32, 
