Four main types of stratified rocks are recognised, and the boy living in a colliery district will have no difficulty in securing specimens of each type from the spoil heaps at the collieries. Such specimens should be searched for and carefully examined. The types referred to are —

1. ARENACEOUS ROCKS, which consist of Sandstones, Gritstones, and Conglomerates. These rocks are made up of rounded water-worn grains, cemented together by siliceous, calcareous, or ferruginous material. The relative size of the component particles fixes the class to which each belongs. When the grains are small, rounded, and of uniform size, the rock is known as a sandstone; when the grains are sharper and less rounded and of slightly variable size, it is known as a grit; and where the rock contains large, rounded, and water-worn pebbles it is described as a conglomerate. The beds, or strata, thus formed, vary from a fraction of an inch to many feet in thickness, and are to be found in all geological formations.

2. ARGILLACEOUS or CLAYEY ROCKS are formed by the fine sediment derived from the aluminous constituents of the igneous rocks. The chief rock of this type is shale, which is merely consolidated mud that has developed a more or less laminated structure, due to its deposition in successive thin layers. Rocks of this type occur plentifully in the carboniferous system, and are usually dark coloured owing to the inclusion of finely divided vegetable material with the mud.

3. CALCAREOUS ROCKS are those which are composed chiefly of carbonate of lime. Many of the calcareous rocks have been formed by the precipitation of carbonate of lime from water carrying it in solution, while others have been formed by the action of living organisms as explained at page 21 et seq. Limestones are easily recognised by the rapid effervescence which takes place when a drop or two of Hydrochloric or Sulphuric Acid is allowed to come in contact with them.

4. CARBONACEOUS ROCKS are derived chiefly from the remains of vegetation. The most important is Coal, which will be described later. See page 22.

Metamorphic Rocks. — The rocks belonging to this group have all undergone alteration since the time of their formation. The chief factors causing metamorphism are heat, pressure, and time, aided by the action of water infiltrating through the rocks. In general, metamorphic rocks belong to the older rock systems, although there are a few exceptions. They were probably in most cases igneous, although in certain cases they may have been stratified, but the changes produced are often so complete as to destroy the former structure and hide the original condition in obscurity. Three types of metamorphism are recognised

1. CONTACT or THERMO-METAMORPHISM. Where great masses of molten rock force their way between strata or along fissures, they bake, harden, and sometimes crystallise the rocks with which they come into contact, thus producing the changes in question.

2. DYNAMIC METAMORPHISM is that due to pressure which, when sufficient to cause movements in the rock mass, generates heat and the combined effects produce startling changes in the original structure. Rocks which have been subject to dynamic metamorphism assume a foliated or platy structure, which gives clear indication of the shear-forces that have been responsible for the change.

3. HYDRO-METAMORPHISM. — When water, either alone or in conjunction with various gases, deposits one class of mineral and takes up another into solution, we get what is called replacement. This process is slowly going on in most rocks by the action of infiltrating water, and where the changes so produced are extensive the action is described as hydro-metamorphism.

Plutonic and Intrusive Rocks. — In addition to forming large masses or bosses, the plutonic rocks may thrust themselves in long, narrow tongues into the surrounding rocks, in which case such tongues are regarded as intrusive. It is therefore advisable to describe the chief rocks which belong to both types under one head.

GRANITE. — This is a crystalline rock of fine, even texture, but in some cases containing large crystals of felspar set in a ground mass of smaller ones. Felspar is the predominating mineral, but it also contains quartz and either muscovite, or biotite mica. In some cases the mica is accompanied, or almost wholly replaced, by hornblende. In such cases the rock is named a hornblende granite.

SYENITE resembles granite, and is closely related to it both in origin and appearance. The chief minerals are Orthoclase and Plagioclase felspar, biotite mica and hornblende.

GABBRO is a dark-coloured rock of the same texture as granite, and is composed of Plagioclase felspar, Augite, Olivine, and Magnetite.

DIABASE is dark coloured like gabbro and contains the same constituent minerals. In diabase the felspar crystals instead of being broad and of irregularly interrupted outline are long (lathe shaped) and the Augite acts as a filler of the space between them.

PERIDOTITE is a heavy, dark-coloured rock of the same texture as granite, but is nearly devoid of felspar. It contains Olivine, Augite, and Magnetite as the chief constituents, with occasionally red garimets as an accessory mineral.

It should be noticed that the composition of the above representative rocks changes step by step, the felspar decreasing and the ferromagnesium minerals increasing. Moreover, in the first named there is no magnetite, whereas in the last magnetite is an important component. Further, free Silica (Quartz) is one of the constituent minerals of the granite, whereas it is absent in the others.

These changes in the nature of the constituent minerals form a basis for the division of the igneous rocks into Acid, Intermediate, and Basic groups. The Acid group consists of rocks containing free quartz ; the Intermediate, of rocks containing felspar and hornblende; while the Basic rocks contain magnetite as one of their constituents.

All rocks decay under the action of the atmosphere, or 'weather' as it is termed, and the appearance of the weathered surface tells whether a rock is Acid, Intermediate, or Basic.

In an Acid rock the weathered surface is creamy white in colour; in an Intermediate rock the weathered surface is light brown; while in a Basic rock the weathered surface is dark brown. These colours are due to the oxidation of iron, the more iron the darker the colour.

Effusive or Extruded Rocks. — These rocks are, as already stated, the product of volcanic action, having been poured out on the surface as molten lavas. Cooling has been rapid, and because of that the rocks are either glassy or hemicrystalline. They may be derived from the same parent mass as the plutonic types, and have practically the same chemical composition as their plutonic relatives, but crystallisation has been wholly or partly prevented, hence they have quite a different structure and appearance.

OBSIDIAN is a rock rich in silica. It is black and glassy in appearance but often passes by slow gradations into PUMICE, which is just obsidian containing a large number of cavities formed by escaping steam and other gases. The glass is blown into thin walls between the cavities; these become semi-transparent, and a porous, dirty, white coloured rock is formed. Obsidian and Pitchstone, another glassy rock that bears a close resemblance to it, have both practically the same chemical composition as granite, of which they are the glassy form.

RHYOLITE is a light-coloured rock generally containing a few well-formed crystals of felspar and also some crystals of quartz. These crystals are carried in a dull, stony base showing a wavy structure due to flow when in the liquid state. It occurs in sheets and is the volcanic form of granite, to which it is closely related in chemical composition.

TRACHYTE is somewhat similar to rhyolite in general appearance, but has a peculiar grey aspect due to the greater abundance of felspar crystals. It is less siliceous than rhyolite and does not contain quartz crystals. Its chemical composition approximates closely to that of Syenite, of which it is the volcanic representative.

ANDESITE is another rock very similar to rhyolite in appearance, but generally darker in colour, due to the presence of hornblende or biotite or both. The rock usually shows flow structure, and frequently a beautiful parallel arrangement of the small felspar crystals which it contains.

BASALT is a dark-coloured or black basic lava with a structure resembling diabase. It is composed of lathshaped crystals of plagioclase felspar, Augite, and Magnetite, with occasionally some Olivine. The rock is frequently, however, of so fine a texture that its mineral components can only be seen by an examination of a thin section with the microscope. In the rock mass it is often found divided up into hexagonal pillars, as at the Giant's Causeway and Fingal' s Cave. A coarser form frequently found as an intrusive rock is known as DOLERITE. These basic lavas are often full of cavities formed by bubbles of steam. The cavities may often become filled or partly filled with secondary minerals deposited from solution in water which percolates through the mass; a cavity so filled is called an Amygdule, and a rock containing such filled-in cavities is described as Amygdaloidal.

Many of the older lavas have undergone considerable alteration by a process called devitrification. The glassy ground mass gradually changes by a process of slow crystallisation until it assumes a dappled stony appearance.

The following table shows the relationship between the various rocks described :–

Crystalline		Hemicrystalline	Glassey
Acid Inter- mediate	Granite (Sp. Gr. 2.65)	Rhyolite (Sp. Gr. 2.5)	Obsidian (Sp. Gr. 2.35)
	Syenite (Sp. Gr. 2.75)	Trachyte (Sp. Gr. 2.5)	Pitchstone (Sp. Gr. 2.5)
	Diorite (Sp. Gr. 2.85)	Andesite (Sp. Gr. 2.75)	...
Basic	Gabbro (Sp. Gr. 2.95)	Basalt (Sp. Gr. 2.9)	Tachylyte (Sp. Gr. 2.7)
	Periodite (Sp. Gr. 3.2)	Dolerite (Sp. Gr. 2.9)	...

It will be noticed from the table that the density of the rock increases as the basic constituents increase, and is lower where the amount of silica is greatest. It is also noteworthy that the crystalline rocks are denser than the corresponding hemicrystalline, while the glassy varieties are the lightest of their class. The densities given are average densities, as rocks are not definite compounds but mineral aggregates, and hence their densities vary a little in different specimens.