
134 | - GEOGNOSY. [Book IL — 
perfectly schistose arrangement. When this takes place, the rock is 
called “ gneissose” or gneiss-granite. (See Book IV. Part vii.) 
Differences in the proportions or nature of the component 
minerals have likewise suggested distinctive names. Of these the 
following are the more important: Granitite,—a mixture of pink 
orthoclase and abundant oligoclase with a little quartz and some 
blackish green magnesia-mica; Protogine,—consisting of orthoclase, 
oligoclase, hexagonal tables of a dark green mica, and pale green 
talc, occurs among the crystalline rocks of the Alps; Syenite-granite, 
—a rock in which hornblende is added to the other normal consti- 
tuents of granite, is usually poorer in quartz than normal granite. 
It derives its name from Syene in Upper Egypt, whence it was 
obtained anciently in large blocks for obelisks and other architec- 
tural works. The well-known Egyptian monoliths are made of it. 
Syenite-granite is found in the Vosges, at Pilson in Bohemia, in the 
Pyrenees, and in different parts of Scotland, notably in masses of — 
tertiary age which have invaded and altered the Lias rocks of Skye 
and Raasay. It there sometimes assumes a porphyry-structure. 
Granulite is by some authors included among the granites (p. 125). — 
Surrounding large masses of granite there are usually numerous 
veins which consist sometimes of granite and sometimes of varieties 
of quartz-porphyry. There can be no doubt that these porphyritic 
protrusions really proceed from the crystalline granite mass. Lossen 
has shown that the Bode vein in the Harz has a granitoid centre 
with compact porphyry sides, in which he found with the microscope 
a true glassy base." Sometimes the rocks associated in this way 
with granite differ in composition from the main granite. Thus 
greisen is a granular aggregate of quartz and mica (usually lepidolite) 
which by addition of felspar passes into granite; Touwrmaline-rock or 
schorl-rock, is a crystalline aggregate of quartz and black tourmaline 
or schorl. 
Granite weathers chiefly by the decay of its felspars. These are 
converted into kaolin, the mica becomes yellow and soft, while the 
quartz stands out scarcely affected. The granite of the south-west 
of England weathers to a depth of twenty feet or more, so that it can 
be dug out with a spade. 
Granite occurs (1) as an eruptive rock, forming huge bosses, © 
which rise through other formations both stratified and unstratified, 
and sending out veins into the surrounding and overlying rocks, 
which usually show evidence of much alteration as they approach 
the granite ; (2) connected with true volcanic rocks (as in the case 
in Skye just cited) and forming, perhaps, the lower portions of 
masses which flowed out at the surface as lavas; and (3) in the heart 
of mountain chains and elsewhere, interbedded with gneiss and other 
metamorphie rocks in such a manner as to suggest that it is itself a 
final stage of metamorphism. Granite is thus a decidedly plutonic 
rock; that is, it has consolidated at some depth beneath the surface, 
* Z. Deutsch. Geol. Ges. xxvi, (1874) p. 856. 
