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138 | _S“GEOGNOSY.. |.) Bane 
hornblende, to which plagioclase, quartz, and mica are occasionally _ 
added. The word, first used by Pliny in reference to the rock of 
Syene, was introduced by Werner as a scientific designation, and 
applied to the rock of the Plauenscher-Grund, Dresden. Werner 
~ afterwards, however, made that rock a greenstone. Tue base of all 
syenites like that of granites is thoroughly crystalline, without an 
amorphous ground-mass. | 
The typical syenite of the Plauenscher-Grund, formerly described 
as a coarse-grained mixture of flesh-coloured orthoclase and black 
hornblende, containing no quartz, and with no indication of plagio- 
clase, was regarded as a normal orthoclase-hornblende rock. 
Microscopical research has, however, shown that well-striated tri- 
clinic felspar, as well as quartz, occur in it. Its composition is: 
—silica, 59°83; alumina, 16°85; protoxide of iron, 7°01; lime, 4°48; 
magnesia, 2°61; potash, 6°57; soda, 2°44; water, &c., 1:29; total, 
101:03. Average specific gravity 2°75 to 2°90. 
Among the accessory minerals of common occurrence may be 
mentioned titanite (sphene), quartz, apatite, epidote, orthite, magne- 
tite, pyrite, zircon. The predominance of one or more of these 
ingredients has given rise to the separation of a few varieties under 
distinctive names. Zvrcon-syenite, the characteristic rock of 
Laurvig in Southern Norway, consists of orthoclase, zircon, horn- 
blende, and the ancient form of nepheline termed eleolite. When 
mica occurs in abundance the rock is termed mica-syenite. Sometimes 
augite in crystals or crystalline granules makes its appearance and 
forms augite-syenite. The name foyate (from Mount Foya in the 
Portuguese province of Algarve), méasccte (from Miask), détrodte (from 
Ditro in Transylvania), are syenitic rocks containing eleolite and 
other minerals. 
Syenite occurs of many different ages from early Paleozoic up to 
Miocene, under conditions similar to those in which granite is found; 
it has been erupted in large irregular masses, especially among meta- 
morphic rocks, as well as in smaller bosses and veins. It is likewise 
sometimes associated with syenitic granite, quartz-porphyry, and 
other orthoclase rocks at the roots of volcanic hills, as in Raasay 
and Skye inthe West of Scotland, where it has overflowed Jurassic 
rocks, and is itself of Miocene age. 
Orthoclase-Porphyry (Quartzless-porphyry) stands to the syenites 
in the same relation that quartz-porphyry does to the granites. It 

is composed of a compact porphyritic ground-mass with little or — | 
no free quartz, but through which are usually scattered numerous 
crystals of orthoclase, sometimes also a triclinic felspar, black horn- 
blende and glancing scales of dark biotite. It contains from 55 to 65 
per cent. of silica, thus differing from quartz-porphyry and felsite in 
its smaller proportion of this acid, but the distinction is one which, 
except by chemical or microscopical analysis, must often be difficult 
to establish between the fine compact felsites and the orthoclase por- 
phyries, especially when the latter (as the microscope shows) contain 
