ROCK-FORMING MINERALS 3 
be split or cleaved more readily in some directions than in 
others. In certain cases cleavage takes place in one direction 
only; in other cases there are two and sometimes three 
directions in which a mineral may be 
more or less readily divided. Separa- 
tion along such cleavage-planes is some- 
times effected with facility, as in mica, 
gypsum, calcite, and fluorite—the cleav- 
age-planes having smooth and lustrous 
surfaces. In other cases, cleavage may 
be more or less imperfect or even un- 
recognisable. When force is applied to 
minerals of this kind, therefore, they 
do not separate along planes, but break 
with an uneven or irregular fracture. 
Quartz is one example, breaking, ASAE (ys oe aie ee 
does, with a shell-like (conchoidal) frac- QUARTZ. 
ture. In its purest form the mineral is a ae eae 
water-clear, and has a vitreous lustre. 
It is infusible before the blowpipe, and insoluble either in 
hydrochloric, sulphuric, or nitric acid. 

Quartz occurs in several ways:—I. Frequently it is a product of 
igneous fusion, being met with as an original constituent of many 
kinds of eruptive rock, such as granite, quartz-porphyry, rhyolite, etc. 
2. It is a not less important ingredient of many schistose rocks, such 
as gneiss, mica-schist, etc., and is thus the result of metamorphic 
action—the nature of which will be considered in a subsequent chapter. 
3. Quartz occurs also as a deposition from aqueous solution, and as 
such has a very wide distribution. Silica deposited in this way is 
derived chiefly from the chemical decomposition of rock-forming silicates. 
Such solutions, percolating through the rocks of the earth’s crust, have 
brought about manifold changes. Frequently, for example, we find 
quartz replacing the original constituents of rocks. Again, many more 
or less loosely-aggregated rocks have been permeated by siliceous 
solutions and converted into hard, unyielding masses. Thus, loose sand 
has been solidified into sandstone, while sandstone, in its turn, has been 
highly indurated and changed into quartz-rock. Another result of the 
circulation of such solutions has been the filling-up of cracks, fissures, 
and cavities of all shapes and sizes, in almost every kind of rock. 
Hence, quartz frequently appears in the form of ramifying veins and 
veinlets, and is one of the commonest minerals associated with ores 
in lodes. 4. As quartz resists decomposition and is the commonest of 
all rock-forming minerals, it enters conspicuously into the formation 
