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> Parr Il. § vi.] CRYSTALLINE ROCKS—MASSIVE. 139 
_ free quartz. This rock is sometimes termed syenite-porphyry, since 
it is associated with syenite much in the same way that elvanite is 
with granite. But this name should be retained for the finely 
crystalline varieties, which would thus represent among the quartz- 
less orthoclase rocks granite-porphyry in the quartziferous series. 
The term Minette (Mica-trap) is applied to a variety which 
contains abundant scales of mica. Orthoclase-porphyry occurs in 
veins, dykes, and intrusive sheets. Probably many so-called 
felstones, whether occurring as lavas or as intrusive masses, among 
the older Palzeozoic formations are really orthoclase-porphyries. 
The orthoclase-porphyry of Pieve in the Vicentin was found by 
~ Von Lasaulx to have the following composition. Silica, 61:07; 
alumina, 18°56 ; peroxides of iron and manganese, 2°60 ; potash, 6°83; 
soda, 3°18; lime, 286; magnesia, 1:08; carbonic acid, 1:36; loss, 
2:13—specific gravity, 2°59. 
Orthoclase-porphyry is largely developed among the later 
Paleozoic formations of Thuringia, the Harz, and Saxony, where it 
occurs both intrusively in dykes, and intercalated in large beds. 
Trachyte.*—A term originally applied to modern volcanic rocks 
possessing a characteristic roughness (rpaxvs) under the finger, is now 
restricted toa rock consisting essentially of sanidine, with more or Jess 
triclinic felspar, usually with hornblende, biotite, and magnetite, and 
sometimes with augite, apatite, and tridymite. It is thus distin- 
guished macroscopically from liparite or quartz-trachyte by the 
absence of quartz. Microscopically it is to be discriminated from 
that rock by the absence or feeble development of the micro- 
felsitic substance so abundant in liparite, and by the preponderating 
aggregate which it presents of minute colourless felspar-microliths 
with usually needles and granules of greenish hornblende and much 
diffused magnetite dust. The average composition of trachyte may 
be stated thus:—silica, 60°?0—640; alumina, 17-0; protoxide and 
peroxide of iron, 6-°0—8-0; magnesia, 10; lime, 3:5; soda, 4:0; 
potash, 2°0O—2°5. Average specific gravity, 2°65. 
Trachyte is an abundantly diffused-lava of Tertiary and Post- 
tertiary date. Jt occurs in most of the volcanic districts of Europe 
(Siebengebirge, Nassau, Transylvania, Bay of Naples, Euganean 
Hills.) It has been poured out upon a vast scale in the western 
territories of the United States. It occurs also in New Zealand. 
Phonolite (clinkstone).°—A term suggested by the metallic 
ringing sound emitted by the fresh compact varieties when struck, 
is applied to a compact grey or brown quartzless mixture of sanidine 
and nepheline with hornblende and usually nosean. Under the 
microscope the ground-mass is not vitreous or half devitrified, but 
1 Z. Deutsch. Geol. Ges. xxv. p. 320. On “mica-traps” see Bonney, Q. J. Geol. Soc. 
Xxxv. p. 165. 
? On trachyte see Zirkel, Micro. Petrog. p. 143. King in vol. i. of Explor. 40th 
Parallel, p. 578. 
$ Boricky, “‘ Petrograph. Stud. Phonolitgestein. Bohmens”—Archiv. Landesdurch- 
| forschung Bohmen. 1874. 

