202 - GEOGNOSY AND GEOLOGY. 
A. Granite Rocks. 
Granite has often permanent characters over considerable extents. 
It everywhere exhibits the same grain, the same color, &c.; on its 
confines, however, variations are sometimes met with; it becomes 
porphyritic, incloses foreign minerals, among which are schorl and 
pistacite (thallite), exhibits a weathered exterior, and is often colored red 
by oxyde of iron. Granite frequently forms masses of great extent, as the 
Riesengebirge and Erzgebirge of central Europe, and sometimes occurs in 
a more isolated condition, as in the Brocken of the Hartz. It sometimes 
penetrates in between abnormal masses, as of gneiss, and frequently forms 
veins both in normal and abnormal rocks. It not unfrequently happens that 
granite is traversed by a newer granite; this has then, in most cases, a 
coarser grain and a different color from the old. The cleavage of granite 
is frequently very decidedly parallelopipedal, being most clearly exhibited in 
weathering, where the blocks present an appearance not dissimilar to a sack 
of wool. These blocks are sometimes tabular, as also globular, and 
combined with a concentrically scaly cleavage (on the Rehberg in the 
Hartz). The rock faces of granite are exceedingly picturesque and 
_ imposing when the mass is of great amount. The valleys then appear like 
deep fissures, with the sides presenting a most magnificent appearance. Th/ 
mountain forms are most generally spherical in outline, with the abov/- 
mentioned bag-like rocks strewn around in every direction on the summis; 
needles and peaks are sametimes exhibited under similar circumstances. / 
Granite, upon the whole, is very rich in veins, the contents of whichare 
very various. Some contain mineral substances resembling one or mop of 
the natural constituents, as feldspar and quartz; some, again, are occipied 
by a newer granite, by syenite, porphyry, greenstone, trap, and jasalt. 
Foreign substances are frequently met with in these veins, as dum) veins 
(these without metallic ores) filled with barytes and fluor spar; or veins, 
with gold inclosed in quartz or masked by sulphuret of iron; silfer and 
silver ores, with galena, specular iron, hematite, oxyde of mangajese and 
tinstone, tungsten, apatite, mispickel, &c. These veins not seldon extend 
into normal masses, or are found at the confines of the two. / 
The weathering of granite is very noteworthy, and furnishes yoduets of 
great importance both to agriculture and the arts. The fedspar, for 
instance, is decomposed by the continuous influence of the atnosphere, of 
carbonic acid, and of water. The crystalline portions are cbthed with a 
loose, soft, opake, dull crust, which sinks deeper and deeyfr, gradually 
transforming the entire feldspar. The increasing volume exertsa mechanical 
influence on the granite in crumbling it to pieces, this taking place first at 
the sharp corners and edges of the cleavage, and subsequenly penetrating 
still deeper. The feldspar thus affected, will, on examinatpn, be found to 
have been partially converted into a bisilicate of potassa, by{he combination 
of an additional quantity of silicic acid from the quartz/ the bisilicate is 
more readily soluble in water. The alumina, with the djoinished amount 
632 
