First of all, we must place a little of our sand upon a piece of black paper and examine by reflected light. We shall soon notice that sand is anything but the simple gritty looking substance it appears on the sea shore. At least four different substances are certain to be seen; if the sand be clean, we shall see bright glassy crystals of quartz; angular, sharp-edged pieces of flint; minute, flat, glistening plates of mica and the broken remains of shells. Should our sand be taken from a spot in the vicinity of volcanic rocks, it will probably contain opaque pieces of magnetite; we may not recognise these when we see them, but they may be separated from the rest of the sand by means of a magnet, to which they will adhere. Probably our sample of sand will be dirty or stained with iron, in this event we may wash it with weak acid, then after drying, we may examine by transmitted light and the various crystals will show up well.
Let us take some clay as our next example. Clay is the substance from which the rocks known as shale and slate have been formed. We cannot see much of the structure of clay with our microscope so let us wash it and, by so doing separate it into its components. We must put a little clay into a tumbler of water and stir it vigorously for some little time, then still stirring, if we pour off the muddy liquid into another tumbler, we shall find that our old friend sand has settled to the bottom. Clay then is merely mud and sand, but we must not throw this sand away without examining it for, very frequently, beautiful minute fossils are to be found amongst it. Little creatures dwelling in the mud, become buried in it and, as more and more mud is formed above them, and it becomes partly solidified in the form of clay, they become fossilized.
If now we examine a piece of sandstone with our pocket lens we shall find that it very closely resembles the sand we have already studied. It may be so soft that we can break it up in our fingers, then if we examine the powdered sandstone beneath the microscope we shall need to be experts to tell whether we are examining sand or sandstone. Should our rock be too hard to break up in our fingers the addition of a little weak acid will reduce it to sand. We see now that a large proportion of sand and a small proportion of mud result in the formation of sandstone, whereas if the proportions are reversed and we have more mud than sand the resulting rock is known as shale or slate. In shale we very frequently find fossils, in sandstone rarely, for the reason that structures buried in the mud, destined to become shale, are protected from the atmosphere and the action of water; sandstone, on the other hand, is porous, moisture trickles through it and any delicate structures which may have been buried in it are more likely to decompose than to become fossilized.
Limestone is easy to obtain and may occupy us for a few moments. We do not wish to go into technical details, but we may say that the word limestone includes a number of rocks which differ largely in appearance and to a considerable extent in composition. It would, perhaps, be more correct to say that there are several kinds of limestone. Some kinds are made up almost entirely of shells and very interesting they are as microscopic objects. One may wonder how a geologist can state with certainty that some spot, may be many miles from the sea, was once covered by salt water. One of these shell-formed limestones may give him the information; he knows that certain shells comprising the rock must have belonged to marine animals, he knows too that whole mountains, which these rocks sometimes form, are not carried bodily on to dry land, so the obvious inference is that the rock was formed below the sea.
The softer limestones may easily be crumbled and powdered for examination under the microscope; the harder kinds should be treated with acid. When acid is added to limestone a considerable effervescence takes place, for the acid decomposes a substance known as calcium carbonate, which the limestone contains, and bubbles of gas are given off. When the limestone has ceased to effervesce the portions of the rock which remain, may be carefully washed in water, dried and examined under the microscope. We shall find that the substance we examine consists of sand containing a goodly number of plates of mica and there may also be a number of sponge spicules. We must bear in mind when we are examining the remnants of limestone after treatment with acid, that shells are largely composed of calcium carbonate, so it is useless to look for any shell remains, for the acid will have dissolved them.
Calcium carbonate, in a nearly pure state, will and does form rock-like structures; we are most of us familiar with stalactites which are formed on the roofs of caves. These structures are usually composed of calcium carbonate though sometimes, notably in some of the Derbyshire caves, Barium takes the place of Calcium. Should we have the chance of examining a section of a stalactite we should certainly do so. We can see the rings which are formed, as layer after layer of calcium carbonate is deposited, in fact the section of a stalactite bears a striking resemblance to a stem of a tree and has, before now, been mistaken for a fossil stem.
If the study of rocks appeals to us we should make a point of examining all the specimens we can lay hands upon. Many quite common specimens may easily be obtained; rock-salt, for example, though in itself not of great interest as an object for the microscope, will readily dissolve in water, leaving behind an insoluble residue of iron which is well worth examination. The majority of rocks, however, are not affected by water and but little by acids. With such specimens the only course open to the microscopist is to prepare sections.
The making of rock sections is certainly different to the cutting of plant or animal sections. It is a laborious business as the enthusiast will find to his cost. The professional makers of rock sections have special grindstones or lathes for the purpose and, even so, the process is not rapid. The amateur must needs do all his preparation by hand. The requirements, in addition to the piece of rock of which we require a section, are a small square of plate glass, some Canada Balsam, emery powder of various grades and an unlimited stock of patience. If possible we choose a piece of rock with one side as nearly as possible flat; this is merely to save labour; the piece of rock should be roughly about half-an-inch square. As a start we rub the flattest side in a mixture, practically a paste, of coarse emery powder and water. As a matter of fact, we may keep the piece of rock in our pocket and grind it when occasion offers on a flat stone wall or on any surface that will assist in producing a flat surface. When we have ground our surface flat and smooth, we finish it off with fine emery powder and may then polish it with jeweller’s rouge. So much for the first side and, if we do not cry enough at this period, we may proceed to the grinding of the other side. Taking our slab of plate glass, we fasten the polished side of our piece of rock to it by means of Canada Balsam, then we may rest for a few days while the Balsam sets. As soon as the rock is firmly fixed to the glass we proceed as before, but in this case the final grinding and polishing must be very carefully carried out. Towards the end of the operation our original lump of rock will be reduced to the thinness of a cover slip and to its fragility. Having given the finishing touches with jeweller’s rouge, we put the glass, with its attached rock section into a bottle of xylol (to be obtained from any chemist). The xylol dissolves the Canada Balsam and the rock section falls from its support. A further washing in clean xylol should be given and then the section is ready for mounting, which is best done in Canada Balsam. The section may be carefully fixed to the slide with a drop of Balsam or it may be covered with a cover slip, in the usual manner.
In our [concluding chapter] we give hints on slide making and addresses of firms who supply slides. Our advice in that chapter is to prepare one’s own slides where possible; in the case of rock sections, however, we must change our advice, only the microscopist of unlimited leisure can find time to make his own slides.
One of the most interesting branches of rock study deals with the fossil remains of plants and animals. Fossils are interesting in themselves: they are doubly interesting because they tell us more than we could ever have discovered without them, concerning the living forms which inhabited the earth at different periods. Geologists know the order in which the various secondary rocks were formed and by studying the fossils of these various rock formations, from the earliest to the latest, can tell which animals and plants have been longest upon the earth. Some of the living forms of to-day have existed from very early times; we know, for example, that cockroaches were upon the earth long ages ago, for their fossil remains are found in the very early rocks.