The Mineral and Geological Kingdoms.
The structure of rocks and the formation of crystals will be found to furnish an endless supply of instructive material for the microscope. In sciences of pure observation, as those of mineralogy and geology, the facts to be observed are of several different kinds, and where so many observers are at work all over the world, constant progress will necessarily be made, as well as continued correction required from change and improvement in the methods of observation. It would be impossible to give even a slight sketch of what has been done in the two departments of nature referred to during the past few years. Mineralogical and geological research have derived very great advantage from having been assigned to professional teaching. But, as Professor Bonney reminds us, the progress made in geological work in particular, has been directly due to the revelations of the microscope. It called forth an instrument of special construction for the purpose, the petrological microscope ([Fig. 79]), well equipped with Nicol’s prisms, and numerous other appliances demanded for the important investigations.
“Upon the history of the two main groups of rocks the microscope has thrown much light. For the igneous rocks it has simplified their classification and determined their mutual relations; while for the rudimentary group, it has shown the true nature of their constituents, and pointed out the sources from which they were derived. But it is in helping to elucidate the problem of the metamorphic rocks, of which much less was known, that the microscope has been of the most service. It has likewise greatly assisted in the attempt to determine the history and mutual relation of these rocks. One of the most important results within the last few years has been the demonstration that without exception these crystallin schists are very old, all probably older than the first rocks in which traces of life have been found. The conclusion arrived at, is that “the environment necessary for changing an ordinary sediment into a crystalline schist existed generally only in the earliest ages, and but very rarely and locally, if ever, since palæozoic time began.”
The crystalline schists then are the relics still preserved to us of the early days of the earth’s history, when the temperature near the surface was still high. Since that time the zone for marked mineralogical changes has been continually sinking, until at the present day it has reached a depth practically unattainable. “The subterranean laboratory still exists, but the way to it was virtually closed at a comparatively early period in the earth’s history.” Greater progress has been made since the microscope was pressed into the service of geology, and inspires the hope that we shall yet learn something more of the earliest ages, when the mystery of life began.
“It may be regarded as one of the most remarkable results of geological science, that an acquaintance with organic forms is at least as necessary for a geologist as a knowledge of minerals, and that a correct knowledge of organic remains (portions of fossil plants and animals) should prove a more certain and unerring guide in unravelling the structure of complicated districts of countries, than the most wide and general acquaintance with inorganic substances. The cause of this, however, is obvious, as the mineral substances produced at any one period of a vast succession of ages, do not appear to have had any essential difference from those formed under like circumstances at another. The animals and plants, however, living at one period of the earth’s history were widely different from those living at other periods. There has been a continuous succession of different races of living beings on the earth following each other in a certain regular and ascertainable order, and when that order has been determined, it is equally certain that we can at once assign to its proper period of production, and therefore to its proper place in the series of rocks, any portion of earthy matter we may meet with containing any one, or even any recognisable fragment of one, of these once living beings.”
The method of preparing sections of minerals and rocks for microscopic examination will be found at pp. 241, 307-309. The sections, it is almost needless to say, must be prepared thin enough to permit the use of transmitted light, as well as for that of polarised light: that is to say, they should range from about 1⁄100th to 1⁄1000th of an inch. Almost any lapidary will cut sections of any choice specimen.[88] The formation of crystals, and the method of preparing them for examination, has also been fully explained in the chapter on polarised light, pp. 219 et seq., and illustrated on [Plate VIII]. It is well known in micro-chemistry that “almost every substance, simple or compound, capable of existing in the solid state, assumes, under favourable conditions, a distinct geometrical figure, usually bounded by plane surfaces and having angles of constant value.
Much useful information may be gained upon micro-crystallography, as well as on almost everything having any relation to the technique of the microscope, in the “Journal of the Royal Microscopical Society.” To the June number (1898) Mr. T. Charters White contributes an article on crystals, and reminds us that the presence of much or little moisture will modify and alter forms, as much and as often as varying degrees of temperature. At the same time he offers a few useful suggestions for the purpose of securing better results, for which purpose he employs hippuric acid, hydroquinine, and picric acid alone or in combination with hippuric acid, and an aqueous solution of bichromate of potassium, crystallised in a tolerably thick emulsion of gum arabic. This is the only aqueous solution; the other solvents have been methylated spirit, acetone, and absolute alcohol, taking these three solvents as types of the greatest volatility, because in making certain crystals it is necessary that the solvent should evaporate quickly, otherwise the crystals will assume their original forms. It is further desirable to make saturated, or even super-saturated solutions of the three chemicals named, as the colours produced under polarised light are of a deeper and richer character than they are if made from weaker solutions. Of the three chemicals named he prefers hippuric acid, for reasons stated, that it is the most manageable, and allows of more time being taken in modifying the formation of the crystals. It is also advisable to slightly warm the glass slide before the drop of fluid is applied. On the whole, picric acid appears to furnish a greater variety of crystals when used in combination with bichromate of potassium and a solution of gum arabic.