23 
The latter are the most important, and are found of re¬ 
markable beauty in Upper Egypt (Fig. 8) ; they were 
formerly, by the ancient Egyptians, frequently used for 
monumental pillars and buildings, and even for statues 
and sarcophagi, and serve for similar purposes to this day. 
Particularly large crystals are found in the southern Tyrol; 
red porphyry is, however, widely distributed in the Schwarz- 
wald, the Erzgebirge, in the Bogese, and in the Caucasus. 
It furnishes an excellent material for the construction 
of roads and pavements. The green porphyry appears 
principally in the transition rocks, as, for example, in 
Norway; it derives its colour from green hornblende, and is 
frequently changed by diminution of the felspathic granules 
into common diorite and greenstone , and if it loses its 
granular structure, into aphanite. The finest antique green 
porphyry (Fig. 6) is obtained from Greece; it receives a 
magnificent polish, aud is used for artistic purposes, for 
the decoration of churches and palaces. The brown and 
black porphyry (Fig. 7), which derives its colour from black 
hornblende, is found of the finest quality at Elfdal in 
Sweden, where it is used in a similar manner. 
Figs. 9 and 10.— Labrador Felspar, Calcareous 
Felspar, Labradorite. 
This mineral likewise crystallises in oblique rhomboidal 
prisms of 94° and 86°, and admits of cleavage in the 
direction of the basal planes; the splintered fragments 
present on two sides a remarkable parallel striation, and 
exhibit in another direction play of colours from grey to 
green, yellow, and red (Figs. 9 and 10). The planes of 
cleavage are remarkably uneven, splintery, and of resinous 
lustre. The hardness is = 6°, the specific gravity 2-68—2*76; 
the calcareous felspars are, therefore, the heaviest of all. 
The chemical constitution of real Labrador felspar from the 
Labrador coast is—silicate of lime with silicate of soda to 
two equivalents of tri-silicate of alumina. Ca 3 (Na 3 ) Si 2 
+ 4 A1 Si. It is decomposed by concentrated acids, and 
the silica is separated. Before the blowpipe it melts like 
potash felspar ; it imparts to the flame, however, a remark¬ 
able yellow-red colour, especially if the test be dipped in 
oil or tallow. The finest felspar, with iridescent play of 
colours, as in the two examples figured, comes from the 
island of St. Paul’s, and from the mainland of Labrador, in 
large partially rounded fragments. It is found, however, 
in Finland and in the syenite of Saxony, in many dole- 
rites, in the gabbro and hypersthene rocks, but without 
play of colours. It is used, when polished, for boxes, 
stones for rings, brooches, etc. 
The anorthite, a felspar from Vesuvius, crystallises in 
similar, but more distinctly marked oblique rhomboidal 
prisms of 110° 57' and 69° 3', and presents a remarkably 
vitreous lustre. It also contains, in addition to silicate of 
lime, silicate of magnesia, and a little potash, but it is 
decomposed by acids, and is found principally in the 
remains of Vesuvius, and at the Somma. 
The petalite is a klino-rhomboidal or triclinic, and the 
triphane or common spodnmen, a ldino-rhombic or mono¬ 
clinic lithium felspar, both of which, before the blowpipe, 
distinctly give the purple red colour of lithium, and melt 
somewhat more readily than the other felspars. The 
former is colourless and of vitreous lustre, the latter is 
greenish and translucent. Found at Sterzing in the Tyrol, 
near Dublin, and in other localities. 
IY. MICACEOUS MINERALS. 
These are distinguished by their low degree of hard¬ 
ness (1’0—2*5), by a remarkable cleavage in thin plates in 
one direction, and by pearly metallic lustre. The thin 
plates have a strong play of colours, are flexible, sometimes 
elastic, and difficult to break, hence they impart to rocks 
a certain firmness and toughness, and an arrangement of 
schistic structure, so that they mostly form peculiar schistic 
rocks, such as gneiss, mica-slate, talc, and chlorite schists. 
All of them are more or less easily fusible before the 
blowpipe; they also agree with the felspathic minerals in 
relation to their composition, so far as they are combina¬ 
tions of silicate of alumina with the monobasic silicates, 
potash, soda, lithium, magnesia, protoxide of iron, etc. 
In general, they owe their colour to the silicate of the pro¬ 
toxide or oxide of iron and manganese. 
Fig. 11.— Pennine, Rhombohedral Mica. 
Pennine crystallises in acute rhombohedrons, some¬ 
times with truncation of the vertex, as seen in Fig. 11 ; 
the crystals can be split, in the direction of their basal 
planes, into very fine flexible laminae; it is also found in 
foliated masses. The colour is green or yellowish-brown, 
inclined to black ; fine plates of it are transparent. The 
hardness, is greater than that of chlorite and talc, and 
exactly the same as in mica (=2’5), the specific gravity 
is 2 - 62—2*64. The constituents are silicate of alumina, 
with silicate of magnesia and protoxide of iron, and 12*58 
of water, the formula being— 
Mo- 3 ) 
. ° > Si 2 + Al Si 2 + 7 Mg H. 
Fe 3 i 
It is therefore a hydrous magnesian mica, which in a 
heated test-tube gives off water ; before the blowpipe it 
melts to a blackish bead without colouring the flame. It 
is found in remarkable quantity in the neighbourhood of 
Wallis, near Zermatt, at Zinnelgletscher, and at several 
places in the Tyrol. 
Figs. 12 and 13.— Common Mica, Lithium and Potash 
Mica, Biaxial Mica, Muscovy Glass, Cat’s Silver 
and Cat’s Gold, Muscovite Mica. 
The primary form is an oblique rhombic prism of 60° 
and 120°, the directions of the plates or laminse are only 
parallel to the basal planes; six-sided tables (Fig. 13) are, 
