_Iron pyrites._--This is a sulphide of iron of very common occurrence. Its crystalline form is cubical. When broken, it emits a sulphurous smell. The brass-yellow coloured cubes so often seen in roofing-slates are familiar examples of the mode of its occurrence. But it is also frequently found in masses having a crystalline surface. 16. SULPHATES.--Only two sulphates may be noticed--namely, _gypsum_, which is a sulphate of lime, with its varieties, _selenite_, _satin-spar_, and _alabaster_; and _barytes_, a sulphate of baryta. _Barytes_ scratches easily with the knife, and from its great specific gravity is often called _heavy-spar_. Gypsum is softer than barytes. CARBONATES.--Two of these only need be mentioned: _calcite_ or _calc-spar_, a carbonate of lime, which scratches with the knife, and effervesces readily with dilute hydrochloric acid; and _arragonite_, also a carbonate of lime, but denser than calcite. SILICATES.--These are by far the most abundantly occurring minerals. The species are also exceedingly numerous, but we may note here only a few of the more important. They are composed of silica and various bases, such as lime, potash, magnesia, soda, alumina, &c. _Augite_ or _pyroxene_ is a black or greenish-black mineral, found, either as crystals, which are generally small, or as rounded grains and angular fragments, in basaltic and volcanic rocks. It never occurs in granite rocks. It is brittle, and has a vitreous or resinous lustre. There are a number of varieties or sub-species of augite. _Hornblende_, like augite, also includes a great many minerals. When the crystals are small, it is often difficult to distinguish hornblende from augite. Common hornblende occurs crystallised or massive, and is dark green or black, with a vitreous lustre. It is generally sub-translucent. It usually crystallises in igneous rocks which contain much quartz or silica; while augite, on the other hand, crystallises in igneous rocks which are of a more basic character--that is to say, rocks in which silica is not so abundantly present. _Felspar_ is a generic term which embraces a number of species, such as _orthoclase_ or _potash-felspar_, _albite_ or _soda-felspar_, and _anorthite_ or _lime-felspar_. _Orthoclase_ is white, red or pink, and gray. It is one of the ordinary constituents of granite, and enters into the composition of many rocks. _Albite_ is usually white. It often occurs as a constituent of granite, not unfrequently being associated in the same rock with pink felspar or orthoclase. In syenite and greenstone it occurs more commonly than orthoclase. _Anorthite_ occurs in white translucent or transparent crystals. It is not so common a constituent of rocks as either of the other felspars just referred to. _Mica_: this term includes several minerals, which all agree in being highly cleavable into thin elastic flakes or laminæ, which have a glistening metallic lustre. Mica is one of the common constituents of granite.