25 
we must glance at the granites which are so widely 
distributed in the primary rocks; the more so because 
they are so frequently the sites of other crystallised mine¬ 
rals, already described, as well as of the following ores.— 
Granite is a crystallised granular mixture of felspar, quartz, 
and mica, in which these elements lie, without definite 
order, in cross and transverse aggregations, so arranged, 
however, that externally, the crystallised planes of any of 
them are seldom apparent. The individual elements are 
more easily distinguished by their peculiar structure and 
lustre, and by their colour and hardness; felspar, in 
general the principal element, is recognised by its even, 
crystalline-ladderlike fracture, being difficult to scratch 
with the knife, giving pale sparks with the steel, having 
sometimes a greyish-white, sometimes a reddish (as in 
Fig. 16), and sometimes a brownish-red colour (as in 
Fig. 17) ; quartz is usually only disseminated in small 
irregular grains of resinous lustre and uneven fracture, not 
capable of being scratched by the knife, giving bright 
sparks with the steel, and infusible before the blowpipe; 
mica, sometimes silver-white, sometimes black (Figs. 16,17), 
of a remarkable foliated structure, strong metallic or pearly 
lustre, of little hardness, and extraordinary elasticity of the 
laminae. Granite is not easily decomposed, very solid and 
hard, and is found from the smallest grains to medium and 
coarsely granular, so that the mica and felspar especially, 
sometimes attain the size of several square inches, and stand 
out from the granite, as, for example, in the silver-white 
mica figured in the plate (Fig. 12), from Modum in Sweden. 
Granite belongs to the most widely-distributed rocks of the 
primary formation, and sometimes rises to the highest moun¬ 
tains; it forms broad mountain ranges; it appears, however, 
sometimes in gangues, apparently breaking through the slaty 
rocks, or issuing out of the older stratified rocks. This occurs 
especially in many valleys, and is more fully discussed sepa¬ 
rately in treatises on geology. It was used, on account of 
its durability and beauty, by the ancients principally for 
monumental purposes, such as monolithic obelisks, sarco¬ 
phagi, vases, etc. The granite of Baveno (Fig. 16) is still 
applied to small works of art, and takes on a very fine polish. 
The red granite of Upper Egypt (Fig. 17) was formerly used 
by the ancient Egyptians in the construction of their 
temples and pyramids, and especially for their numerous 
sepulchres, for sarcophagi, and was even worked into statues. 
y. ZEOLITIC 
These essentially resemble in their elements the fel¬ 
spars, in so far as they generally occur in a crystalline or 
crystallised form, and, with the exception of the leucite, 
are combinations of silicic acid with alumina and the alkalis. 
They may be distinguished from them, however, by their 
considerable percentage of water, and by their ready fusi¬ 
bility, which is such that they can be decomposed by acids. 
In general they are colourless, or only accidentally coloured, 
MINERALS. 
and appear principally in volcanic rocks, sometimes in their 
clefts or cavities, sometimes mixed with them or forming 
the greatest part of their bulk ; so that, by the crumbling 
of the rocks concerned, they exercise the same influence 
on the vegetable world as the felspars. The combinations 
of soda and lime, however, play the principal part among 
the alkaline elements, and potash only enters as an im¬ 
portant element into the leucite. 
PLATE VII. 
Fig. 1.—Leucite, White Garnet, Amphigene. 
The leucite crystallises almost always in a trapezoidal 
solid with four-and-twenty faces, which body is for this 
reason also called a leucitoid; the cube with truncated 
angles has also been observed, however, and therefore some 
specimens have the primary form of the cube ; irregular 
and roundish grains are more frequent. The crystalline 
faces appear generally dim or but slightly lustrous, some¬ 
times they are rough, or of grey, yellowish, and reddish 
colours. The fracture is extremely irregular, vitreous, 
lustrous, and conchoidal, according as the mineral has been 
submitted to a white heat or to melting; the planes are 
also seldom in proportion to one another, sometimes several 
crystals are aggregated in a drusy form, or twin-wise. 
The hardness is 6*0—6*5, so that it gives sparks with a 
steel, but on being struck it generally flies in pieces, indi¬ 
cating its vitreous friability. It is decomposed by hot 
acids, and the silica is separated. The powder has an 
alkaline reaction, and colours a decoction of violets green, 
by which it may easily be distinguished from all other 
similar minerals, and especially from the felspars. The 
constituents are two-thirds of silicate of potash with three 
equivalents of silicate of alumina K 3 Si 2 + 3 A1 Si 2 . In¬ 
fusible before the blowpipe, but colouring the flame dis¬ 
tinctly blue. It forms an important ingredient of the older 
lavas of the Somma and of Vesuvius, at the Laacher Lake, 
and in the neighbourhood of Rome. 
Figs. 2 and 3.—Zeolite, Mesotype, Natrolite. 
Zeolite crystallises in right rhombic prisms of 91° 20 / 
and 88° 40 / as its primary form ; in such a way, however, 
that usually the basal edges are truncated, and form the 
combination with the rhombic octahedron (Fig. 3); more 
frequently it occurs in fascicular needles, which are trans¬ 
parent and colourless. Hardness — 5*0—5*5, specific gra¬ 
vity = 2*2—2*25. The natrolite (Fig. 2) appears in semi- 
globular, kidney-shaped forms, of concentrically radiated 
structure, of yellow, reddish, brown, and white colours, so 
that, in a transverse section, or when polished for orna¬ 
mental purposes, it presents star-like markings ; sometimes 
also it occurs in crystalline aggregations, white, rose-red, 
fleshy-red, or brown, with a pearly or silky lustre. The 
constituents are silicate of soda and silicate of alumina, with 
