CHAPTER XVII.

MICROSCOPICAL INVESTIGATION.

FREQUENT reference has been made in the foregoing pages to the advantage of studying minerals and rocks under the microscope. By this means we are enabled to trace the minuter structures of the earth's crust, and to follow many of the stages in the formation of its rocks. We can tell which mineral of a rock crystallized first, and indeed can follow every phase of crystallization, in such a way as to explain many otherwise unknown parts of the history of the rocks. Moreover by this method we can trace the subsequent changes which rocks have suffered, in the chemical alteration of their minerals by percolating water, with the resulting secondary products. While a chemical analysis informs us of the ultimate chemical constitution of a rock, a microscopic analysis brings before us its mineralogical composition, showing in what forms the chemical elements have been combined, and how diverse two rocks may be in structure and texture, though in chemical composition nearly alike.

The field-geologist, however, besides these inquiries, often needs some ready means of determining the nature and petrographical grade of rocks which he cannot

satisfactorily name by any of the usual methods available to him. By far the most valuable aid in this respect is supplied to him in the examination of thin slices with the microscope. He ought to be able to prepare his own slices, though when he can have this satisfactorily done for him he may save time for other work.

THE PREPARATION OF THIN SLICES.

To prepare slices of rocks and minerals for the microscope it is not necessary to procure a costly and unwieldy set of apparatus, nor to go through a lengthened course of training. The operation is facilitated, indeed, by the possession of a machine for cutting thin slices, and for reducing and polishing them when mounted on glass. A machine well adapted for both purposes was devised some years ago by Mr. J. B. Jordan, and may be had of Messrs. Cotton and Johnson, Grafton Street, Soho, London, for £10 10s. Another slicing and polishing machine invented by Mr. F. G. Cuttell, 52, New Compton Street, Soho, London, costs £6 10s. But these machines are rather unwieldy to be carried about the country by a field-geologist. Fuess of Berlin supplies two small and convenient hand-instruments, one for slicing, the other for grinding and polishing. The slicing machine is not quite so satisfactory for hard rocks as one of the larger more solid forms of apparatus worked by a treadle. But the grinding-machine is exceedingly useful, and might be added to a geologist's outfit without material inconvenience. If a lapidary is within reach, much of the more irksome part of the work

may be saved by getting him to cut off thin slices. The thickness of each slice must depend greatly upon the nature of the rock, the rule being to make the slice as thin as the rock will allow, so as to save labour in grinding down afterwards.

Excellent rock-sections, however, may be prepared without any machine, provided the operator possesses ordinary neatness of hand and patience. He must procure as thin chips as possible of the rocks he proposes to slice. These he can usually obtain in the field where he is hammering. He should select as fresh a portion of the rock as may be accessible, and by a dexterous use of the hammer break off from a sharp edge a number of thin splinters or chips, out of which he can choose one or more for making into rock-slices. These chips may be about an inch square. It is well to take several of them, as the first specimen may chance to be spoiled in the preparation. The geologist ought also always to carry off a piece of the same block from which his chip is taken, that he may have a specimen of the rock for future reference and comparison. Every such handspecimen, as well as the chips belonging to it, ought to be wrapped up in paper on the spot where it is obtained, and inside the wrapper should be placed a label or piece of paper with the locality and any notes that may be thought necessary. It can hardly be too frequently reiterated that all such field-notes ought as far as possible to be written down on the ground where the actual facts are before us for examination.

Having obtained his thin slices, either by having them slit with a machine or by detaching with a hammer as thin splinters as possible, the operator may proceed

to the preparation of them for the microscope. For this purpose the following simple apparatus is all that is absolutely needful, though if a grinding-machine be added it will save time and labour.

List of Apparatus Required in the Preparation of Thin Slices of Rocks and Minerals for Microscopical Examination.

1. A cast-iron plate, inch thick and 9 inches square.

2. Two pieces of plate-glass, 9 inches square.

3. A Water-of-Ayr stone, 6 inches long by 24 inches broad. 4. Coarse emery (1 lb. or so at a time).

5. Fine or flour emery (ditto).

6. Putty powder (1 oz.).

7. Canada balsam. (There is an excellent kind prepared by Rimmington, Bradford, especially for microscopic preparations, and sold in shilling bottles.)

8. A small forceps.

9. Some oblong pieces of common flat window-glass; 2×1 inches is a convenient size.

10. Glasses with ground edges for mounting the slices upon. They may be had at any chemical instrument-maker in different sizes, the commonest being 3X I inches.

II. Thin covering-glasses, square or round.

These are sold by

the ounce; ounce will be sufficient to begin with. 12. A small bottle of spirits of wine.

The first process consists in rubbing down and polishing one side of the chip or slice. We place the chip upon the wheel of the grinding-machine or, failing that, upon the iron plate, with a little coarse emery and water. If the chip is so shaped that it can be conveniently pressed by the finger against the plate and kept there in regular horizontal movement, we may proceed at once to rub it