p. xvii. It cannot be scratched with a file, but itself is hard enough to scratch glass easily. It is infusible before the blowpipe; is not acted on by acids, whether cold or hot; and, with the addition of carbonate of soda, is easily dissolved to a glass before the blowpipe. None of the members of the Quartz family have a specific gravity exceeding 2.84.

Calc Spar or Calcite, the next mineral to Quartz as regards its abundance, also assumes a variety of aspects. They may all be scratched with the point of a knife; they all effervesce on the addition of a drop of dilute muriatic acid, and are infusible before the blowpipe, but shine with an intense light, and are rendered alkaline by being converted into quick lime, when the carbonic acid is expelled by heat. When not compact massive, the cleavage is very perfect rhombohedral, and the specific gravity does not exceed 2.8.

The Felspars rarely assume granular forms, and never occur fibrous or columnar, but either in tabular crystals or in cleavable masses. They are commonly colourless, or varying in tint from white to flesh-red, sometimes bluish-green and brown, and have a vitreous lustre, which, in some instances, inclines to pearly. The cleavage of Felspar is highly characteristic: one face of cleavage is perfectly smooth, and another, nearly at right angles to it, is somewhat less perfect. Orthoclase may be recognised from the other varieties of Felspar, by having the two cleavage-planes at right angles to each other.

The Zeolites are most frequently associated together in cavities or cracks in amygdaloidal rocks, though they are sometimes found in granite and other rocks. The various members of the Stilbite group are distinguishable by the pearly lustre of their cleavage. They are likewise remarkable for often assuming laminato-radiated forms, and are frequently acicular or in radiated masses consisting of slender fibres.

Hornblende and Pyroxene are often not easily distinguishable when not in crystals, except by chemical analysis. When crystallized, Hornblende often occurs in six-sided prisms, while Pyroxene has commonly four-sided prisms. They both vary in colour, from white to black through grass-green and olive-green shades. Both are distinctly cleavable, with the exception of Epidote, which has no very distinct cleavage. The crystals and the columnar forms of the latter variety have also a more solid appearance, and present a smoother surface; and, when broken longitudinally, the prisms do not show the cleavage-plane and that splintery look which are observable in Pyroxene and Hornblende under similar circumstances.

Micaceous Minerals consist of every thin and easily separable laminæ. Of these, Muscovite, Phlogopite, Biotite, and Lepidolite are closely related, and possess in common the characters of having their laminæ elastic; of yielding no water (or very little) in a tube; of fusing only at the edges before the blowpipe; of not being acted on by acids; and of affording, with a cobalt solution, sometimes a clear blue, but generally a dull blue tint. The specific gravity of this group varies from 2.75 to 3.3, and the hardness from 15 to 2. Pyrophyllite, Margarite, Euphyllite.-With the exception of the former mineral, the lamina are rather brittle, and the colours are white, or of a pale tint. They all afford water in a tube; are fusible before the blowpipe at their edges (Pyrophyllite swells out); afford little or no action with acids, and give a blue colour with a cobalt solution. The specific gravity varies

from 27 to 31. The hardness ranges between 3.5 and 4.5, except in Pyrophyllite, in which it is only 1.5.

Chlorite, Ripidolite, Clinochlore, and Pyrosclerite.-In these minerals the laminæ are flexible, but not elastic, and they often have a slightly greasy feel.

They afford no water in a tube; fuse at the edges more easily, before the blowpipe, than the preceding group, but give no blue colour to cobalt solutions. They are slightly acted on by acids, giving mostly dark green solutions, except in the case of Pyrosclerite, which is often reddish.

Talc resembles the preceding group in most characters, but the lamina are much softer and more greasy, but are not so thin, and the colour is generally pale green. It is infusible before the blowpipe, and insoluble in acids. With cobalt solution it gives a reddish colour, with some difficulty.

Brucite bears some resemblance to Talc in its whitish and greenish colour, and in being infusible before the blowpipe. When heated in a tube, it gives off water, and is entirely dissolved in acids without effervescence.

Diallage, Bronzite, Hypersthene, Clintonite, Chloritoid, though sometimes approaching to micaceous in structure, are, more correctly speaking, foliated. The lamina are brittle, and not easily separated. Marmolite differs from the above in having a greasy feel, and in bearing some resemblance to Talc.

Gypsum, or sulphate of lime, is very soft, and may be scratched with the nail. It differs from the carbonate (or Calc Spar) in not effervescing with acids. It turns white before the blowpipe and crumbles, but is fused only with difficulty. When crystallized it is generally colourless, often transparent, and separable into thin lamine, which can scarcely be bent without breaking.

Uranite, Red Zinc Ore, and Copper Mica.-The first is of a bright green or yellow colour, and crystallizes in square tabular crystals; the second (an oxide of zinc) is bright or deep red; and the third, a deep green, crystallizing in hexagonal crystals, which give the reaction of copper.

In the following work, under the head of Comp. (Composition), is represented, by means of a formula, the chemical composition of each mineral, supposing it to be perfectly pure or free from foreign admixtures. The meaning of the symbols employed to denote the simple substances forming the components is explained further on, at p. xliv. The chemical formula is (in most cases) followed by the per-centage amount of each ingredient present, on the above-mentioned supposition of their being altogether free from extraneous matter.

Perhaps, however, it should be stated here that each equivalent of oxygen is represented by a dot placed over the symbol of the substance with which it is combined: thus Fe represents the metal Iron; Fe, the protoxide of that metal (or the combination of one atom of the metal with one atom of oxygen); and Fe, the sesquioxide, or peroxide of the same metal, in which three atoms of oxygen are present.

The bar drawn through some of the symbols, as in the above, Te, Al, Mn, H, &c., indicate that the substances they represent are in the state of a sesquioxide; or, in other words, that two atoms of the base (represented by the letters) are combined with three atoms of oxygen (represented by the dots). By using the above form of expression, the symbols are rendered much simpler than would be the case if figures and letters were employed to represent the oxygen.

The letters R and are used to denote one or all of those simple substances which can be substituted for each other in a mineral without effecting any essential change in the outward form of the crystal, and which are, therefore, said to be isomorphous with each other (i. e. to possess similar forms). These substances are Iron, Manganese, Lime, and Magnesia.

Sulphur has, in a few instances, been represented by a dash placed above the symbol; thus Iron Pyrites, or Bisulphide of Iron, may be represented either by FeS2, or by

Fe.

The small figures in the formulæ imply that they only refer to the symbol which they immediately follow: thus, in 3AlSi, the small figure 2 placed after Si means that it applies only to the Silica; while the figure 3 placed before the formula denotes that it has reference to all the succeeding symbols, which, written in full, would then become 3l + 3Si2, or 3Al + 6 Si. When symbols are joined, it means that they are in a state of chemical combination: thus Sial denotes that the silica and alumina are combined in the form of silicate of alumina.

CLASSIFICATION OF MINERALS.

WHEN the first difficulties of the science have been overcome, and the student has acquired a sufficient knowledge of minerals to be able to recognise them by their characters and properties-the next step is that of classification -or their arrangement into classes, families, and species.

To accomplish this in a satisfactory manner is a task of considerable difficulty. Each author, in consequence, seems to consider himself at liberty to recommend a system of his own-the result of which is that numerous modes have been proposed by different writers, of various degree of merit; some natural, others artificial, and some, again, partaking of a sort of compromise between these two extremes.

This has led to much confusion, and a highly unsatisfactory state of things. It is partly in consequence of the want of agreement between the various authors who have treated on this branch of the subject, and the practical inconvenience of a purely chemical arrangement (combined with some other motives relating more especially to facility of reference), that the author has been led to the adoption of an alphabetical form for the present work.

But although it has now become a recognised principle, that chemical composition must constitute the basis of any really perfect system of classification, an arrangement founded solely on chemistry is practically attended with much inconvenience. In some instances, the adoption of an artificial system, or some modification of one, may be found useful. For example, in a collection of minerals intended to illustrate some special purpose, as the application and use of minerals in the arts and in jewelry, what are commonly known by the name of gems and precious stones might with propriety be allowed to occupy a prominent position, and be formed into a group by themselves, as was proposed by Allan. On a similar principle, collections illustrative of the mineral resources of our own and Foreign Countries (such as those which will form a part of the proposed International Exhibition of 1862) will convey more information to the mind, and be of much more use as objects of comparison, if each metal be made to form a separate group; the ores of iron, of copper, and of lead, for instance, being all placed by themselves.

On the same principle, other modifications of previous systems may be devised to suit particular cases, or as necessity may require.

The system of classification proposed originally by Berzelius, and adopted at the British Museum, is founded upon the Electro-Chemical theory. This, in many cases, leads to a great amount of inconvenience in practice. The minerals of the various metals, for instance, are by this means, dispersed and widely separated from each other-occasioning much confusion to the stu

dent, and involving considerable loss of time in tracking the ores of each particular metal through the various Cases amongst which they are distributed. A far more generally useful mode of arrangement, and one recommended by its greater comparative simplicity for working purposes, is that according to which the following List of Minerals has been drawn up. It is based on chemical composition, with the introduction of such modifications only as have been considered likely to increase its practical utility. At the outset it commences by making a broad distinction between the metallic and non-metallic minerals; thus dividing minerals into two sufficiently well marked groups. The former, again, are subdivided into four subordinate classes, and the latter into five; the different members composing each of which are allied to one another by mutual affinities. Thus it will be perceived that each metal, with its Ores and Salts, constitutes a group by itself; the latter being formed with reference to the component bases of the Minerals, and not to the Acids.

The general principle of the classification here laid down is that followed by Mr. Warington Smyth in his Lectures on Mineralogy at the School of Mines.