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To avoid the long grinding that must occur when chips are used, professional workers always employ in the first instance a lapidary's cutting disc, which is an iron disc "armed" from time to time by forcing diamond-powder into its edge. The diamond is mixed with a little olive oil and placed with the finger on the edge of the disc. The disc is then slowly rotated while a surface of flint or agate is pressed against it, the tiny splinters of diamond thus becoming set in the iron like the teeth of a circular saw. A well armed disc will cut perhaps some thirty ordinary sections, and must be re-armed directly it ceases to give a clean cut and a sharp hissing noise when in motion.

The disc is preferably placed horizontally and rotated by a band working from a wheel driven and controlled by hand. The specimen is held in a clamp and is drawn against the disc by a cord passing over a little pulley; this cord is weighted at the end according to the severity of the work to be performed. The disc must be continuously lubricated by a little stream of soap and water kept running on to it from the tap of a vessel set above it. A wisp of soft string may be conveniently made to hang from the tap and to rest its free end on the disc, so as to act as a guide for the descending drops.

By such a machine thin and even large slices can be prepared, which require little grinding to transform them into microscopic sections. Sections, moreover, can be cut from minerals in special directions, by careful adjustment of the specimen in the clamp.* Small specimens can be held by being cemented on to a block of wood by means of the "electric cement" described on p. 21, the wood being then fixed in the clamp. Friable objects, such as pumice, must be saturated in hot balsam in a dish over a spiritlamp or Bunsen-burner and allowed to become thoroughly cool. They may then, with care, be cut or ground like other materials.

Finally, it may be useful to mention that emery can be obtained in London for about 1s. 6d. per 7 lb. packet. Diamondsplint, which must be powdered in a steel mortar after purchase, costs about 5s. per carat, two carats lasting a considerable time. Plate-glass, suitable as a basis on which to perform grindings, may be got in about 18-inch squares from any glass-cutter at a moderate cost. The iron discs used in lapidary's work are supplied by Messrs. Cotton & Johnson, of 14 Gerrard Street, Soho, London, W. Most amateurs, however, will find a machine rather a luxury than a necessity, considering the small number of sections they will require in the course of any year. While * For a delicate method, see Stöber, Bull. Acad. Roy. de Belgique, 3e. série, t. xxxiii. (1897), p. 843.

it is of very great importance to have practice and confidence in the making of rock-sections, it is fortunate that now in some towns, especially in Germany, professional lapidaries are springing up, ready to relieve the geologist of work that is in the majority of cases tedious.

That the application of the microscope to the study of rocks is by no means a modern development has been alrea ly seen in our review of Cordier's classic investigations. Some older authors, notably Delesse, derived great advantage from the examination of polished surfaces of rocks under the microscope; and, such surfaces being commonly procurable in all countries, whether among the débris of ancient Rome or the workshops of Indian artists, it is of considerable profit to have studied at least a typical selection. Though polarised light cannot be employed, there are in some cases actual advantages over a thin section, inasmuch as the rock can be often examined by a lowpower objective for a considerable depth, and the tridimensional character of the various objects and structures becomes realised. As a brilliant example of results gained in this manner, we may cite the plates and text of Delesse's Recherches sur les Roches globuleuses, a memoir that should be looked at in this connection by all to whom libraries are accessible.

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While it is now well for the beginner to study carefully a series of rock-sections under the microscope, these should never be considered apart from the rocks from which they have been After understanding, with the assistance of the sections, the main points of structure, an immense amount of work can be done by powdering up a rock, sifting where necessary, and examining the fragments on a microscopic slip, first by reflected light, and then mounted in water or balsam under a cover-glass. The methods of Cordier (pp. 110-113) must never be forgotten, since the expert in the use of microscopic sections is apt sometimes to lose sight of the form of the solid mineral, and of all but its optical characters. The powdered rock naturally lends itself to the isolation and complete testing of its constituents (p. 113).

SOME WORKS ON PETROGRAPHY AND ROCK-FORMING MINERALS. FOUQUE and LÉVY. Minéralogie Micrographique; Roches éruptives françaises. 2 vols., text and plates. Quantin, Paris, 1879. (The coloured plates should be carefully studied.)

HARKER. Petrology for Students. 2nd ed., Cambridge, 1897. (Especially dealing with microscopic characters, and thoroughly modern.) * Mémoires de la Soc. géol. de France, 2me. serie, t. iv., p. 301.

HUSSAK. Anleitung zum Bestimmen der Gesteinbildenden Mineralien. Engelmann, Leipzig, 1885.

JANNETAZ. Les Roches. Rothschild, Paris, 1884. (Coloured plates of microscopic sections.)

LÉVY and LACROIX. (1) Les Minéraux des Roches. Baudry, Paris, 1888. (Contains an admirable plate showing the application of Newton's Colour-Scale to mineral-sections in polarised light.) (2) Tableaux des Minéraux des Roches. Baudry, Paris, 1889. (Reference tables founded on above work.)

MERRILL. Rocks, Rock-weathering, and Soils. Macmillan, 1897. (Regards rocks from a true geological aspect; excellent illustrations.)

ROSENBUSCH. (1) Mikroskopische Physiographie. 2 vols., 3te. Auflage. Koch, Stuttgart, 1896. (With photographic plates of minerals as seen in sections.) (2) Hülfstabellen zur mikroskopischen Mineralbestimmung. Koch, Stuttgart, 1888. (Reference-tables, translated by Hatch. Sonnenschein). (3) Elemente der Gesteinslehre. Nägele, Stuttgart, 1898. (An excellent general manual.)

ROSENBUSCH and IDDINGS. Microscopical Physiography of Rock-making Minerals. 4th American edition, revised. Wiley, New York; Chapman & Hall, London, 1900. (The photographs in the original work of Rosenbusch are reproduced.)

RUTLEY. (1) Rock-forming minerals. Murphy, London, 1888. (A compact guide, including the higher applications of the microscope.) (2) Granites and Greenstones. Murby, 1894.

TEALL. British Petrography. Dulau & Co., London, 1888. (An invaluable work both for the text and the exceptional character of the coloured plates.)

ZIRKEL. Lehrbuch der Petrographie. 2te. Auflage, 3 vols., Leipzig, 1894. (The most complete work on rocks extant, dealing with their wider aspects as well as microscopic structure The section on rock-forming minerals is equally admirable.)

CHAPTER XVI.

THE MORE PROMINENT CHARACTERS TO BE OBSERVED IN
MINERALS IN ROCK-SECTIONS.

Ir is well known that some minerals, such as magnetite, are opaque even in the thinnest sections yet prepared. Hence their characters must be studied by reflected light, which may conveniently be concentrated on their surface by an ordinary condenser on a stand, or by fitting up a pocketlens so that it can be moved about upon an upright rod. The annexed sketch (fig. 16) is a suggestion for such a contrivance; a stout brass wire is fitted into a steady base, and a large cork slides up and down upon it. A second wire, also thick, so as to present a good surface, is bent at right angles; one end is thrust through the cork and the other carries a second cork, projecting out sideways, which can thus be moved in three directions. A part of this cork is cut away so as to form a slit, into which the handle of the lens fits and in which it is held by tightness. The same sort of arrangement Fig. 16. may be useful for holding a lens in various positions when small loose objects are being examined by this instrument alone.

The characters that can be studied in opaque minerals are the form of the sections, the colour and lustre by reflected light, the cleavages (which often appear as black lines or cracks) and the products of decomposition that occupy the cracks or surround the crystal.

The characters of translucent minerals may be separately treated as follows :-

Form. In rock sections allotriomorphic crystals, or those that are bounded merely by abutting against neighbouring forms, are so common that the worker is at the outset apt to feel discouraged. The idiomorphic crystals, with their proper boundaries, in which he has so delighted during his mineralogical studies, are found to be, as it were, interesting curiosities when compared with the mass of the crystalline substances with

which he has to deal in rocks. But he soon realises that sufficient traces of regular outline frequently remain from which he can build up some well-known form; while in the lavas a number of sections are so sharp and satisfactory that he can determine the angles between various planes, compare one result with another, and picture to himself the complete crystal from some ten or a dozen scattered sections in a slide. For the correct appreciation of the objects thus seen under the microscope a knowledge of the solid forms of minerals and mineral-aggregates is obviously necessary; the study of "micropetrology" becomes otherwise something artificial, extra-natural, and is liable to be looked on with hesitation by workers accustomed to rock-masses in the field.

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Just as no mountain-mass can be described by a stranger from a number of hand-specimens, however beautiful, so no rock can be adequately described from isolated microscopic sections. Again and again the observer will pass from his section to the solid specimen, and from this, in memory at any rate, to the great mass of which it formed a part. Even in his choice of descriptive terms, he will remember that nature is tridimensional, and that the object regarded by him under the microscope as a "plate" or "lath" may be of considerable thickness in a direction perpendicular to the section.

Having determined by a series of optical tests that certain sections in a slide belong to the same mineral, some fair deductions may be made as to the nature of the solid form. A fundamental enquiry is, however, how has the rock-section been cut with regard to any structural peculiarity of the mass? Thus a foliated rock will yield elongated and even wisp-like sections of its minerals if cut perpendicular to the foliation-planes, while more or less expanded lenticular forms will appear in slides prepared parallel to these planes. A rock, again, in which the minerals have been arranged by flow may ordinarily show a number of prismatic outlines; yet a section perpendicular to the direction of flow may represent the prismatic rods as square or almost circular areas, suggestive of cubes or granules. If, finally, the minerals so arranged are prisms, the three axes of which differ considerably in length, the majority of sections may appear as fairly short "laths" in one slide, as long ones in another, and as broad plate-like forms in a third. The annexed diagrams (fig. 17) will aid in making this matter clear.

When, however, the rock-specimen presents no peculiarities of structure, when in all aspects it looks reasonably similar in constitution, a single slide will enable one to ascertain the

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