ROCK-FORMING MINERALS Y 
Corundum (Al,O;) crystallises in hexagonal prisms and pyramids, 
but is often massive. It is a very hard mineral (9 in the scale), and has 
a high specific gravity (3-9 to 4). It occurs sparingly in some granites, 
syenites, schists, metamorphosed limestones, and basalts. The beautiful 
clear-coloured varieties (Ruby and Sapphire) are highly valued as gems. 
As a rock-ingredient corundum is of little importance ; sometimes, how- 
ever, it occurs in considerable masses. Amery (an intimate mixture of 
corundum and magnetite and hematite) occurs in veins and layers in 
crystalline schists. 
Pyrolusite, Psilomelane, and Wad (oxides of manganese) are also un- 
important rock-formers, but they often appear (particularly psilomelane) 
as thin films coating the walls of cracks and fissures or the surfaces of 
bedding-planes in various kinds of rock. The films often assume plant- 
like forms (“dendritic markings,” see Plate XXVIII.). The earthy 
varieties of these oxides occasionally form bedded masses. 
LE SEELEC ATES 
FELSPAR GROUP 
Felspar is a general term for a number of closely related 
minerals which play a very important rdéle as rock-formers. 
They are the chief constituents of most eruptive rocks, and are 
met with likewise more or less abundantly in many crystal- 
line schists. They vary in colour, but are usually grey, white, 
or reddish; occasionally, however, they show yellow, green, 
or blue tints. As rock-constituents they frequently assume 
the form of tabular crystals, or appear as long rods or 
rectangular lath-shaped bodies. All are characterised by two 
well-marked sets of cleavage-planes (at, or nearly at, right 
angles) which show usually a glassy or pearly lustre; 
further, all have approximately the same hardness (6 to 7), and 
specific gravity (2-54 to 2-76). Chemically, they are silicates 
of aluminium with either potassium, sodium, or calcium, 
or several of these together. Hence we have potash 
felspar, soda felspar, lime felspar, soda-lime felspar, etc. 
These felspars so closely resemble each other that it is often 
hard or even impossible to distinguish one from another 
by the unassisted eye. This, of course, is especially the 
case when the crystals are small. Usually, however, the 
particular class or series to which a felspar belongs can 
be determined by examination in thin slices under the 
