KINDS OF ROCKS. 443 
times nephelite, haiiynite, tridymite. Apatite exists in the 
rock in microscopic forms, and there is also more or less of 
the rock in a glassy state. Sometimes contains augite, and 
has a higher specific gravity. Quartz-trachyte has often 
nearly the composition of granite in which there is little 
mica. Hruptive only. 
VARIETIES.—The two principal divisions under each, trachyte and 
quartz-trachyte, are: A. Sanidin-trachyte, in which the mass is chiefl 
Sinidin ; and B. Oligoclase-trachyte, or Domyte, in which it is partly 
oligoclase ; but the two graduate into one another. Both occur por- 
phyritic with tabular crystals of feldspar; and in the latter (as at 
the Drachenfels) the tables are sanidin. They graduate into vesicular 
or scoriaccous trachyte. 
Trachyte, according to Reyer and Suess, occurs in the region of the 
Kuganean Hills of Tertiary, Cretaceous and Jurassic age; and the 
felsyte of Paleozoic age is often hardly distinguishable from it, while 
identical with it in composition. 
Trachyte and quarte-trachyte graduate also into felsyte-like volcanic 
rocks of like constitution, porphyritic or not so. The latter sometimes 
shades into rocks of semi-glassy nature called 
14. Pearlstone, when somewhat pearly in lustre; PITCHSTONE 
when haying a pitch-like lustre; and these into the glassy 
voleanic material called Obsidian. These glassy rocks often 
contain spherules which are concretions consisting of feld- 
spar with some quartz. Pumice is a light, porous, feldspa- 
thic scoria, with the pores capillary and parallel. Ordina- 
ry obsidian, that consists chiefly of feldspar, and is hence 
nearly free from ir on, belongs here; the rest of it belonging 
with the augitic igneous rocks. 
15. Leucityte. —A grayish rock consisting chiefly of leucite 
in a felsitic state, with disseminated leucite crystals. Occurs 
at Point of Rocks, Wyoming Territory, according to King 
and Zirkel. It differs from amphigenyte, in containing no 
pyroxene, or only traces of it. e 
3. MICA AND SODA-LIME-FELDSPAR ROCKS. 
I, NOT CONTAINING NEPHELITE. 
1. Kersantyte.— Fine-grained ; consists of biotite and white 
oligoclase, with some quartz, and frequently some horn- 
blende in needles, and magnetite ; sometimes porphyritic. 
Analysis afforded Delesse 58-0 per cent. of silica, and 63°88 
of silica for the oligoclase. From Visembach and St. Marie- 
aux-Mines in the Vosges, the Saxon Erzebirge, and Nassau. 
