In conclusion, it may be re-urged that the young geologist must bear in mind that to collect specimens is the least part of his labour. If he collect fossils, he cannot go wrong; if he be so fortunate as to find the bones of any of the higher animals, he will, in all probability, make an important discovery. Let him, however, remember that he will add greatly to the value of his fossils by labelling every single specimen, by never mingling those from two formations, and by describing the succession of the strata whence they are disinterred. But let his aim be higher: by making sectional diagrams as accurately as possible of every district which he visits (nor let him suppose that accuracy is a quality to be acquired at will), by collecting for his own use, and carefully examining, numerous rock-specimens, and by acquiring the habit of patiently seeking the cause of everything which meets his eye, and by comparing it with all that he has himself seen or read of, he will, even if without any previous knowledge, in a short time infallibly become a good geologist, and as certainly will he enjoy the high satisfaction of contributing to the perfection of the history of this wonderful world.

ARTICLE XII.

FOURTH DIVISION, SECTION 2.

MINERALOGY.

BY THE LATE SIR HENRY DE LA BECHE, C.B., F.R.S., &c.

Revised by W. H. MILLER, Esq.,

Professor of Mineralogy in the University of Cambridge.

A GLANCE at the best treatises on mineralogy, even those wherein the matter is most condensed, is sufficient to show that a profound acquaintance with this science can only be acquired by careful study, and by means of a competent knowledge of certain other sciences, the aid of which must be obtained properly to comprehend the internal and external structure and chemical composition of minerals. The naval man may nevertheless accomplish much, more especially respecting the mode of occurrence and probable origin of minerals under certain conditions, and he may also add by his researches to the catalogue of known substances of this class, and may discover new varieties of known minerals.

To classify the natural substances described under the head of mineralogy, very various methods have been adopted, chiefly, however, divisible into those based upon their external characters or chemical composition. The best classification hitherto proposed appears to be that of G. Rose, in his work entitled Das Krystallo-chemische Mineralsystem: Leipzig, 1852.' It is recommended by the great general resemblance of the minerals constituting each chemical group.

The fifth edition of Naumann's Mineralogy, published

in 1859, contains a list of 636 minerals, supposed really to differ sufficiently to be so separated, independently of many merely considered as varieties, or accidental. It will be obvious that a voyager, especially when his general time may be occupied with other duties (only a portion of it applicable to mineralogy, and that irregularly), cannot expect to make himself familiar with all these substances. With many of those more commonly found he will have little difficulty, and by practice he will readily detect them when presented to his attention. Those which form the constituents of rocks it is especially necessary to learn and distinguish, since so much of geological importance often turns upon their proper determination. Those which are referable to the useful class should engage his attention, since while, on the one hand, valuable ores of the useful metals and other important substances are often neglected (even in our mining districts unusual though valuable ores have been thrown away at no remote times); on the other, many a mineral, commercially worthless, is treasured up, often even to the neglect of those of high value, some particular brilliancy of appearance or fancied resemblance to precious or metallic substances having misled the collector.

However desirable it may be to consider inorganic matter as a whole, the conditions under which its parts have been found to combine either naturally or artificially being only regarded with reference to the general subject, so that the natural bodies, commonly termed minerals, merely constitute a portion of this whole, it is important that the voyager be enabled to distinguish natural minerals, both as respects science and its applications. If he possessed no other means of distinguishing minerals from each other than chemistry afforded him, he would in many instances, from the want of the needful space and appliances on board ship, have the extent of his mineralogical labours greatly abridged. At the same time, with a box containing certain chemical substances, a slight apparatus, and a blowpipe, he will, after a little practice, find his power to distinguish minerals chemically far greater than he might at first anticipate.

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For an account of the characters of minerals, and the methods of observing them, the voyager is referred to any of the following treatises :-Blum, Lehrbuch der Oryktognosie: Stuttgart, 1854.' Dana, 'System of Mineralogy: New York and London, 1854.' Dufrénoy, Traité de Minéralogie Paris, 1856.' Haidinger, Handbuch der bestimmenden Mineralogie: Wien, 1851.' Kengott, Tabellarischer Leitfaden der Mineralogie: Zürich, 1859.' Naumann, Elemente der Mineralogie: Leipzig, 1859.' Nicol, Manual of Mineralogy: London, 1849.' Phillips, Elementary Introduction to Mineralogy: London, 1852.' Quenstedt, Handbuch der Mineralogie: Tübingen, 1854. The characters of greatest importance in the discrimination of minerál species are-form, cleavage, fracture, lustre, colour, streak, hardness, specific gravity, and the chemical reactions. Hardness, lustre, colour, and streak are easily observed, yet, being frequently alike in different minerals, and not admitting of being measured with much accuracy, they cannot be considered as exact determinative characters. Specific gravity is an important distinctive character, and admits of being observed with exactness. The observation, however, which requires the use of a delicate instrument, must be made very carefully, in order to be of value. The form, cleavage, and the chemical reactions, lead with most certainty to the determination of the species of a mineral.

On breaking a crystallized mineral, it generally exhibits a tendency to split in the direction of one or more planes, called cleavages. In many cases the blow of a small hammer is sufficient to produce the cleavage: in other cases the mineral, supported upon a block of wood or metal, must be struck by the point of a punch, or the edge of a small chisel, having its edge nearly in the direction of the cleavage, driven by a hammer. The cleavage of a small bit of a mineral may be obtained by pressing it between the edges of a wire-nippers. It is found that in the same mineral the cleavages are always disposed in the same manner, forming constant angles with each other, and with the faces of the crystal.

Instruments called goniometers have been invented for measuring the angles which the faces (including the cleav age-planes under that term) make with each other. By comparing the observed angles with the angles recorded in books on Mineralogy, we either arrive at once at a knowledge of the species of the mineral, or else ascertain that it belongs to one of a more or less limited number of species. For this purpose the cleavages are usually more useful than the natural faces, because, being brighter, the angles they make with each other can be more easily and more accurately measured, and also because, being fewer in number, there is little difficulty in identifying them with the cleavages recorded in the descriptions of mineral species.

Carengeot's goniometer, used by Hauy, is incapable of affording results sufficiently accurate; Wollaston's reflective goniometer is consequently the only instrument the use of which can be recommended. A full description of the method of observing with it is given in Phillips's 'Mineralogy,' edition of 1852. It is a common mistake to suppose that crystals seldom occur with faces sufficiently bright for measurement with this instrument. When a small screen, at a distance of from 10 to 20 feet from the observer, having in it a hole of a square inch in area, through which the light of the sun is reflected from a plane mirror, is used for the bright signal, it is difficult to find crystals which have not faces bright enough to allow the angles they make with each other to be measured with very tolerable accuracy. The flame of a good lamp or candle serves very well for the bright signal, but is much inferior to sunlight. Descriptions of other contrivances for measuring the angles between the faces of crystals will be found in the Sitzungsberichte der Mathematische-Naturwissenschaftlichen Classe der k. Academie der Wissenschaften in Wien,' vols. xiv. xvii.; in 'Silliman's Journal' for September, 1857; and in the 'Philosophical Magazine' for July, 1858.

A knowledge of elementary crystallography sufficient for the observer's purpose may be gained from the chapter on that subject usually given in mineralogical treatises, or