Page:Popular Science Monthly Volume 9.djvu/452

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THE POPULAR SCIENCE MONTHLY.

microscopically examined. When the granite rocks become decomposed, as they often do in Cornwall and elsewhere, through the wear and tear of the weather, we frequently find the disintegrated materials so separated that the silicate of alumina of the feldspar forms thick deposits of the beautiful white clay known as kaolin, and which is so valuable to the china-manufacturer.

The mica of granite is usually a variety called Muscovite, or potash mica; this again on chemical analysis is found to contain, as did the feldspar, silica, alumina, and potash, and also often some iron and manganese. There are several different sorts of mica, also, sometimes found in granite, especially Biotite, the composition of which varies from the above; but all the micas may be known by their being found in flatfish crystals, which may be split up into an infinity of thin leaflets. Thus far our unaided eyesight and the help of the chemist have

PSM V09 D452 Orthoclase and plagioclase feldspar.jpg
Fig. 2. Orthoclase Feldspar. Fig. 3.—Plagioclase Feldspar.

shown us what granite is made of; but we are now beginning to learn that, would we know something of the real history of a rock, a far minuter examination is needful, and geologists are rapidly learning that they must turn to the microscope if they would receive answers to many important questions, both as to the history and also as to the composition of rocks. A marvelous light has been shed during the past few years on rock-structure through this minute investigation, especially with the aid of polarized light. The intricacies of the closest-grained rocks have been disentangled, their component parts distinguished from each other, and the very order and history of their combination in the mass revealed. Now, when we examine our granite beneath the microscope, which can be done by having thin slices prepared, we shall learn something about it which we could hardly hope to have discovered without this aid. There has been much speculation as to the origin of granite, whether it is a plutonic—that is, an old volcanic rock—or whether it is only a deposit from water consolidated and altered during the lapse of long ages by heat and pressure: the microscope will help us to the truth. When magnified and examined with the polariscope, a thin section of granite is a very beautiful object, and its different constituent parts stand revealed with the greatest distinctness: we at once learn to see the crystals of