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 , but it may be remarked here that once formed they undergo all the foldings, faulting and disturbances which have affected the sedimentary rocks of other kinds.

.—The following are general works on the deposits of the useful minerals, in addition to Posepny’s volume already mentioned: In English—J. A. Phillips, revised by Henry Louis, Treatise on Ore-Deposits (London, 1896); J. F. Kemp, Ore-Deposits of the United States and Canada (New York, 1900); Prime’s translation of Von Cotta’s Ore-Deposits (New York, 1870); H. Ries, Economic Geology of the United States (New York, 1906); W. H. Weed’s translation of Beck’s The Nature of Ore-Deposits (New York, 1905); Genesis of Ore-Deposits (American Institute of Mining Engineers, 1901); G. P. Merrill, The Non-Metallic Minerals (New York, 1904). In German—B. von Cotta, Die Lehre von den Erzlagerstätten (Freiberg, 1859); A. von Groddeck, Die Lehre von den Lagerstätten der Erze (Leipzig, 1879); R. Beck, Lehre von den Erzlagerstätten (Berlin, 1904); A. W. Stelzner and A. Bergeat, Die Erzlagerstätten (Leipzig, 1905–1906). In French—E. Fuchs and L. de Launay, Traité des gîtes minéraux et métallifères (Paris, 1893); G. Moreau, Étude industrielle des gîtes métallifères (Paris, 1894).

 MINERALOGY, the science which describes and classifies the different kinds of mineral matter constituting the material of the earth’s crust and of those extra-terrestrial bodies called meteorites. The study of minerals is thus a branch of natural history, but one in which certain of the exact sciences find an application. The determination of the composition and constitution of minerals is a chemical problem; their optical and other physical properties are determined according to the principles of physics; the study of their crystalline form and structure belongs to crystallography; their modes of occurrence, origins, associations and changes come within the province of geology and petrology; while a consideration of the localities at which they are found requires some acquaintance with geography. Finally, there is the economic side, dealing with the mining and application of useful minerals, the extraction of metals from their ores, and the uses of minerals for building, decoration and jewelry.

In this article we shall treat only of the general characters of minerals; their special characters will be found in the articles on the individual minerals.

History.—Owing to their numerous applications for useful and decorative purposes, minerals have attracted the attention of mankind from the earliest times. The stone and bronze implements of prehistoric man and many of his personal ornaments and charms were directly or indirectly of mineral origin. The oldest existing treatise on minerals is that written about 315 by Theophrastus ( —On Stones, English version by John Hill, 1746), of which only a portion is now in existence. Minerals were then classed as metals, stones and earths. The last five books of Pliny’s Historia naturalis, written about 77, treat of metals, ores, stones and gems. Some of the Arabian philosophers devoted themselves to the study of minerals, and about 1262 Albertus Magnus wrote his De mineralibus. In the 16th century Georgius Agricola published several large volumes, dealing more especially with the mining and metallurgy of metalliferous minerals, in which more exact descriptions were given of the external characters: he mentioned several minerals by names (e.g. blende, fluor, quartz) which are now in common use. About the same period there appeared

the systematic treatise on minerals of K. Gesner (1565), and that on precious stones by Anselm Boethius de Boodt (1609). The remarkable researches of Erasmus Bartholinus on Iceland-spar were published in 1669, and J. F. Henckel’s Pyritologia in 1725. Later came the Systema naturae of C. Linnaeus (1735). Although the importance of chemical properties was recognized by the Swedish chemists—J. G. Wallerius (1747) and A. F. Cronstedt (1758)—the external characters of minerals formed the basis of the mixed systems of classification of A. G. Werner (1774) and of other authors, and even as late as the Natural History System of Mineralogy of F. Mohs (1820).

It was not until the end of the 18th and beginning of the 19th century, when the foundations of crystallography were laid by Romé de l’Isle and R. J. Haüy, and chemistry had assumed its modern phase, that any real advance was made in scientific mineralogy. It was then recognized that chemical composition and crystalline form were characters of the first importance, and that external (natural history) characters were often more or less accidental. During this period numerous mineral substances were analysed by Scheele, Klaproth, Charles Hatchett, Vauquelin, Kirwan, Berzelius, Rose and other chemists, and many new mineral-species and chemical elements discovered. After W. H. Wollaston’s invention of the reflecting goniometer in 1809, exact measurements of the crystalline forms of many minerals were made. The principles of isomorphism and dimorphism enunciated by E. Mitscherlich in 1819 and 1821 respectively cleared up many difficulties encountered in the definition of mineral-species. About the same time also the discovery by E. L. Malus of the polarization of light gave an impetus to the optical examination, by Sir David Brewster and others, of natural crystals. Later, the investigation of rocks in thin section under the microscope led to the exact determination, particularly by A. Des Cloizeaux (1867), of the optical constants of rock-forming minerals.

A distinction is to be made between essential and non-essential characters. Essential characters are those relating to chemical composition, crystalline form, crystallo-physical properties and specific gravity; these are identical, or vary only within certain defined limits, in all specimens of the same mineral-species. Non-essential characters—such as colour, lustre, hardness, form and structure of aggregates—depend largely on the presence of impurities, or on the state of aggregation of imperfectly formed crystalline individuals. In an absolutely pure and perfectly developed crystal all the characters may be said to be essential, but such crystals are of exceptional occurrence in nature, and certain of the characters are subject to modification under different conditions of growth. For example: a well-formed crystal of haematite (“specular iron ore”), with its smooth black faces and brilliant metallic lustre, is strikingly different in appearance from a piece of massive haematite (“red iron ore”), which is dull and earthy and bright red in colour; the former is so hard that it can only with difficulty be scratched with a knife, while the latter is quite soft and soils the fingers. Both specimens will, however, be found on analysis to have the same chemical composition (Fe2O3), the same crystalline structure (as determined by the optical characters under the microscope in the case of the massive variety), and very nearly the same specific gravity (especially if this be determined upon finely powdered material, the effect of cavities being thus eliminated). The essential characters being identical, the difference between the two specimens lies in the state of aggregation of the material: with “specular iron ore” we have a single crystal, while with the “red iron ore” we are dealing with a confused aggregate of minute crystalline individuals, which have interfered with each other’s growth to such an extent that no crystal-faces have been developed. Such differences do