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“jee ae GEOGNOSY. -" [Boomtt: 4 
Hornblende andesite! consists of a triclinic felspar and horn- _ 
blende, often with a little sanidine. The ground-mass is frequently 
quite crystalline, or shows a small proportion of a felsitie nature, 
with microliths and granules. 
Two varieties are distinguished. (1) Quartzferous or Dacite— 
This rock, besides the minerals enumerated, contains augite, magne- 
tite, quartz and apatite in a ground-mass which has a felsitic, some- 
times spherulitic, glassy, or finely granular base. Mean composition, 
silica, 66°10; alumina, 14°80; iron protoxide, 6°30; lime, 5:30; 
magnesia, 2°40; alkalies, 4°70; water, 0°50. Mean specific gravity, 
2°60. (2) Quarizless—This variety, sometimes distinctly crystalline, 
sometimes extremely compact, almost vitreous, contains crystals of 
plagioclase, hornblende, augite, and rarely sanidine, with not 
infrequently biotite, apatite, and tridymite, imbedded in a base com- 
posed of an aggregate of colourless felspar-microliths, and grains of 
magnetite. Mean composition, silica, 50°75; alumina, 17-25; oxides 
of iron, 7°57; lime, 6:0 ; magnesia, 1°30 ; potash, 3°10; alkalies, 4:0; 
water, 1:0. Specific gravity, 2°7—2°8. 
Hornblende andesite is a volcanic rock of Tertiary and Post- 
tertiary date found in Hungary, Transylvania, Siebengebirge, and 
recently ascertained to have a considerable development in some of 
the western territories of the United States. 
Propylite——A name given to certain Tertiary volcanic rocks 
consisting of a triclinic felspar and hornblende in a fine-grained non- 
vitreous ground-mass. They are subject to considerable alteration, 
the hornblende being converted into epidote. Some quartziferous — 
propylites have been described by Zirkel from Nevada, wherein the 
quartz abounds in liquid cavities containing briskly moving bubbles, 
and sometimes double enclosures with an interior of liquid carbon 
dioxide. The best account yet given of this rock will be found in 
Zirkel’s Microscopical Petrography,’ already cited... A specimen from 
Storm Canon, Fish Creek Mountains, gave silica, 60°58; alumina, 
17°52; ferric oxide, 2°77 ; ferrous oxide, 2°53; manganese, a trace; 
lime, 3°78; magnesia, 2°76 ; soda, 3°30; potash, 4°46 ; carbonic acid, 
a trace; loss by ignition, 2°25; specific gravity, 2°6—2:°7. 
Porphyrite.—This term may be used as the designation of rocks 
which consist essentially of some triclinic felspar, and show a true 
porphyry ground-mass containing crystals of plagioclase with mag- 
netite or titaniferous iron, hornblende, augite,or mica. Thus defined, 
these rocks correspond in the plagioclase series to the orthoclase- 
porphyries and felsites of the orthoclase series. Their texture varies 
from coarse crystalline-granular to exceedingly close-grained, and 
passes occasionally even into vitreous. Porphyrite is a volcanic rock 
very characteristic of the later Paleozoic formations, occurring there 
' See Zirkel, Microscopical Petrog. p. 122. King in vol. i. of Explor. 40th Parallel, 
p. 562. 
2 Vol. vi. of the U. S. Haploration of the 40th Parallel, p. 110. See also King in 
vol. i. p. 545, and O. K. Dutton’s “ High Plateaux of Utah” (U. 8. Geographical and 
Geological Survey of the Itocky Mountains), chaps, iii. and iv, 

