442 DESCRIPTIONS OF ROCKS. 
0-10, magnesia 1°42, soda 1°12, potash 6°66, titanium dioxide 
1-91, water 4:13=10022. (Compare with analyses of hy- 
drous muscovite, or margarodite.) Metamorphic. 
11. Felsyte. Quartz-Felsyte. (Huryte, Petrostlex.)—Com- 
pact orthoclase, with often some quartz intimately mixed; 
fine granular to flint-like in fracture; sometimes contains 
oligoclase. Colors white, grayish-white, red, brownish-red 
to black. G.=2:6-2°%. Both metamorphic and eruptiye. 
VARIETIES.—There are two sections, I. Felsyte, and II. Quartz- 
Felsyte, and under each occur the following varieties. a. Porphyritic 
Felsyte, or Porphyry ; containing the feldspar in small crystals distri- 
buted through the compact base ; color red and of other shades ; called 
sometimes Quaréz-porphyry, when the base is a quartz-felsyte. b. Con- 
glomerate felsyte ; containing pebbles, as at Marblehead, Mass., and in 
the White Mountains. c. Oligocluse-bearing ; containing this triclinic 
feldspar intimately blended with the orthoclase. d. Cellular or amyg- 
daloidal. e. Hlvanyte ; essentially a quartzose felsyte, of gray, bluish- 
gray to brown and red colors, and often containing disseminated grains 
or crystals of quartz and feldspar, and some oligoclase ; some compact 
slate-rock has the same composition. Qccurs in Cornwall. 
The metamorphic and eruptive kinds are not easily distinguished. 
The form2r-ocsurs associated with sedimentary strata, and often con- 
tains pebbles or other evidence of fragmental origin ; while the lat- 
ter is frequently in dikes, that is, fills the fissures through which it 
was ejected. Some of the eruptive felsyte has nearly the aspect of 
trachyte, with which rock it is identical in composition. Much of the 
red porphyry coatains hornblende with the feldspar of the basc, and 
has the constitution of dioryte (p. 44/7). 
12. Porcelanyte. (Porcelain Jasper.)—A baked clay, hay- 
ing the fracture of flint, and a gray to red color; it is some- 
what fusible before the blowpipe, and thus differs from 
jasper. Formed by the baking of clay-beds, when they con- 
sist largely of feldspar. Such clay-beds are sometimes baked 
to a distance of thirty or forty rods from a trap dike, and 
over large surfaces by burning coal beds. Metamorphic. 
13. Trachyte. Quartz-Trachyte.—Consists mainly of feld- 
spar, which is partly in glassy crystals, either sanidin or 
oligoclase ; and, owing to the angular forms of the glassy 
feldspar and the porosity of the rock, the surface of frac- 
ture is rough, whence the name from the Greek trachus, 
rough. Sometimes contains disseminated quartz, and is 
then quartz-trachyte Color ash-gray, greenish-gray, brown- 
ish-gray, but sometimes yellowish and reddish. G.=2°d- 
2°7. Besides the feldspar there are distributed, somewhat 
sparingly, through the mass, in many kinds, minute needles 
of hornblende, crystals or scales of biotite, magnetite; some- 
