Minerals. — Unlike most rocks, minerals have a definite chemical composition. They are inorganically formed substances and are with few exceptions distinguished by their crystalline shape, general form, surface colour, hardness, lustre, touch, taste, or odour. A very large number of different minerals are found in nature, but only a comparatively small number are found as constituents of the commoner rocks, and these are usually described as rock forming minerals. So far as rock building is concerned, minerals are usually classified as essential minerals or accessory minerals.

Essential minerals are those whose presence is necessary and implied in the name of a rock. Accessory minerals are those which may or may not occur in a certain rock and whose presence or absence does not sensibly affect the chief characteristics of the rock in question.

Identification of Minerals. — Before giving a short description of the principal rocks, it will be necessary to briefly describe the common minerals that enter into their composition, and to indicate how they may be identified.

CRYSTALLINE SHAPE is often a ready means of recognising minerals, but the question of shape and other properties which enable crystal forms to be recognised is too intricate to be dealt with here.

COLOUR appears to be a ready test, but is by no means a safe one as it is liable to vary in different specimens of the same mineral. Colour in a mineral is usually due to the inclusion of certain metallic oxides, as those of iron and manganese.

The DENSITY or SPECIFIC GRAVITY of a mineral is a very important property, and is simply the weight of a mineral compared with the weight of an equal volume of water. It may be expressed thus

The HARDNESS of a mineral is a very useful test, and is often employed in the field. This test is simply one for ascertaining the power of the mineral to resist abrasion. The degree of hardness is determined by reference to a scale known as Moh's Scale, which is made up from the following minerals:

Any mineral that will scratch one of the minerals in this scale but is itself scratched by the mineral immediately above has a hardness fixed between the two. Thus, if a mineral makes a scratch on a piece of Calcite and is itself scratched by Fluorspar, its hardness lies between 3 and 4. It is well to remember when in the field that Talc and Gypsum can be scratched by the thumb-nail, Calcite by a bronze coin. Fluorspar in turn scratches the coin. Apatite is easily scratched by a good knife. Felspar is marked with the knife with great difficulty, while quartz scratches the knife, or if brought sharply against the back of the knife blade will strike fire from it.

Hardness beyond that of Quartz is rare, and does not occur in the common rock-forming minerals.

LUSTRE is a surface appearance possessed by some minerals in a marked degree. It is called metallic when it shines like metal, vitreous when it appears like glass, resinous when it has an oily appearance, and pearly when it has the appearance of mother-of-pearl.

Touch, taste, and odour only serve to distinguish a very few minerals, and need not be further described.

Most rock-forming minerals when ground down into thin sections are transparent and can be examined under the microscope. Rock sections prepared thus are studied by means of a polarising microscope, and many of the peculiar physical properties of crystals are thus observed. Such studies belong to the more advanced stages of geology, and require an extensive knowledge of Chemistry, Physics, and Mathematics.

Common Rock-forming Minerals.

QUARTZ (SiO₂) is pure silica in a crystalline condition. When fully developed the crystals form hexagonal prisms with a pyramid end. Often, however, in igneous rocks, the form of quartz is quite indefinite. It has a hardness of 7, a specific gravity of 2.6, and shows a well-defined glassy or vitreous lustre. It is usually transparent or milky white, but is sometimes coloured, as in the case of Cairngorm (smoky quartz) and Amethyst (purple quartz).

The FELSPARS are a family of rock-forming minerals with many similar characteristics, and are Silicates of Alumina and either potash, soda, or lime. They are usually creamy white to pink in colour, and show well marked cleavage planes or planes of weakness along which, if struck with a hammer, they tend to separate. In one group, the Orthoclase, these cleavage planes form two sets crossing each other at right angles. In the other group, called the Plagioclase, these cleavage planes cross each other at an oblique angle. They have a hardness of 6, and a specific gravity of from 2.56 to 2.75. The orthoclase felspars show a peculiar structure, often discernible by the naked eye, known as Carlsbad twinning, while the plagioclase felspars show repeated or lamellar twinning.

The MICAS form another group of minerals composed of Silicate of Alumina, and potash or soda, and in addition iron and magnesium as well. Two well-defined types are common, Muscovite or White Mica, where alumina and the alkalies are the chief components, and Biotite or Black Mica, where, in addition to alumina and the alkalies, iron and magnesium are present in quantity. The micas have a hardness of from 2 to 3, and a specific gravity of from 2.76 to 3.2, and are possessed of an extremely perfect cleavage. They are readily split into thin plates, which are easily bent, but show great elasticity, the latter a property rarely possessed by mineral substances.

HORNELENDE and AUGITE are both iron-magnesium silicates, which contain lime and sometimes alumina. They are dark brown to black in colour, though dark green is not an unusual colour for hornblende. They have a hardness of from 5 to 6 and a specific gravity ranging from 2.9 to 3.5. Both are possessed of well-marked cleavage, the planes in the case of the Hornblende forming angles of about 70 and 120 degrees with each other, while in Augite the cleavage planes cross each other nearly at right angles.

OLIVINE is a more basic iron-magnesium silicate than the two last mentioned. It is olive green in colour, and shows a decided vitreous lustre. It has a hardness of 6.5 to 7 and a specific gravity of from 3.2 to 3.5. It occurs in rounded grains, which usually show a tendency to change through taking up water.

MAGNETITE (Fe₃O₄), the magnetic oxide of iron, occurs in small grains and frequently in well-developed Octahedral crystals. The colour is black, and under the microscope the grains are quite opaque — the hardness is from 5.5 to 6.5 and the specific gravity 5. As the name implies, the ore is strongly magnetic, and is readily separated from rock powder containing it by means of a magnet.

Secondary Minerals. — The foregoing minerals are the chief rock-formers found in the igneous rocks, although they by no means form a complete list. A few other minerals which have developed since the rocks themselves were formed are also common constituents of certain rocks. Their development in the rock mass by changes taking place in certain of the original components has given rise to their name, and they are described as secondary minerals. They are for the most part hydrated silicates, that is, silicates that have combined with water.

CHLORITE is a dirty green mineral that splits up into thin flakes like mica, and is a decomposition product of the ferromagnesium silicates due to their having combined with water. The flakes are easily distinguished from mica flakes as they are not elastic.

SERPENTINE is another decomposition product, and is formed by the combination of water with Olivine. Its colour varies, being usually yellow to light green, but is sometimes very dark green, and in a few cases red.

KAOLIN or CHINA CLAY is a hydrated silicate of alumina formed by the decomposition of Orthoclase felspar. In most igneous rocks the change is incomplete and the felspars only show signs of kaolinisation on their surfaces, and along cracks. When pure, the colour is white, but it is frequently stained with other mineral substances also undergoing decomposition.

CALCITE or CARBONATE OF LIME is a white or transparent mineral often found lining cavities and deposited along joints in rocks. It crystallises in the rhombohedral form, and in this state is known as Calcspar or nailhead spar.

The descriptions here given include only a few of the more important minerals, but ability to recognise those described will greatly assist the beginner to determine the various types of rocks that he will find in his practical work.

To this brief description must be added a warning that reading by itself is of little value in the study of minerals and rocks, and every opportunity should be taken to examine specimens both in the school classroom and in nature's classroom — the field.

An extensive examination of the surface of a country will show that the rocks of which it is composed can be divided into three main groups. These groups are easily recognised by their general appearance and typical structure, and are classified as Igneous, Stratified, and Metamorphic; a classification based on their origin.

Igneous Rocks. — The rocks belonging to this group are those that have solidified from a molten liquid magma, and are characterised by their massive, and frequently crystalline or hemicrystalline structure. They show no signs of stratification, and are more or less homogeneous throughout. There are three types belonging to this group: (1) The effusive, which have been poured out on the surface as lava-sheets by volcanic action. (2) The intrusive, which have been injected into fissures or cracks in the overlying rocks and have solidified there. (3) The plutonic, which have solidified at great depths under enormous pressures, and have been exposed at the surface only after long ages of denudation.

Most of the rocks belonging to the Igneous group are dense and hard, and are largely composed of Silica or Silicates. The way in which these rocks occur, whether poured out as lava on the surface or intruded through beds of other rock, has a great influence on the contours and scenery of a country.

Stratified Rocks. — The rocks belonging to this group are formed by disintegration of the older plutonic rocks and the consolidation of the sediment in lakes or on the margin of the sea. Being built up of worn fragments of other rocks, they are easily recognised by their structure, as the conditions under which they have been deposited cause the fragments to be sorted out into beds or layers. This group also includes rocks formed by the accumulation of vegetable and animal remains, and in addition rocks that have been precipitated from solution by chemical agencies. Because of the important part played by water in collecting, carrying, and sorting out the components of these rocks, they are frequently called Aqueous Rocks.