The study of rocks and minerals by means of the microscope is apt to be disappointing. In the first place, to study them seriously we require a special microscope, the ordinary instrument, with which we may poke into the deepest secrets of the animal and plant world cannot translate for us half the story of the rocks. Again, to understand rocks and minerals we must study them somewhat deeply. Geology, as the science of rocks is called, is no more difficult than botany or zoology, the sciences of plants and animals respectively. Botany and zoology, however, appeal in some degree to nearly all of us; we may learn a good deal concerning the structure of the cockroach with the help of our microscope and be interested in the revelations of our instrument, but to embark on a detailed course of the minute internal anatomy of insects would appeal to few of us. Animal or plant life may be studied piecemeal and enjoyed on account of its absorbing interest. Geology must be studied from its very beginnings if we are really to understand what we see beneath the microscope.

In the hand, a lump of rock, say of granite, may be of exceeding beauty. The body of the rock is, perhaps, a delicate pink, scattered here and there are the flat glittering plates of mica and brilliant crystals of quartz. Other rocks, less common, vie with the rare gems for beauty of colouring and lustre. As thin microscope sections these once gorgeous specimens are colourless, dull and, unless we understand them, uninteresting.

There are, however, many mineral substances which we may study with advantage for, if our investigations do not take us very far towards elucidating the story of the rocks, we shall at any rate discover something that is new to us. We may well commence our studies with the examination of ordinary sand. This is not a rock, you will probably exclaim. You are right but one day it may be a rock, it all depends upon circumstances.

Before we take out our microscope let us have a short talk about rocks in general, then we may understand better where we are. Rocks of one kind and another make up the crust of the earth, that is pretty obvious anyway. Thousands and thousands of years ago, how many we are not prepared to guess, this old earth of ours was a sphere of molten rock. Needless to say it was far too hot for any plants or animals to dwell upon it. Very, very gradually the outer crust cooled down and in time it became sufficiently cool to support animal and vegetable life. Then there were rivers and seas and then came, from time to time, rain and wind and frost and even earthquakes. The earthquakes cracked the crust of the earth, moisture entered the cracks and, when the frosts came, pieces of rock were broken away, owing to the expansion of the moisture in the cracks as it became converted into ice. The rain and wind helped to carry the broken pieces of rock, ever downwards towards the sea, but before the sea was reached the big boulders became broken up, by their buffeting, into shingle and sand and mud. In the course of long ages, longer than it is easy to imagine, these broken pieces of rock, gathered together as we have seen from various districts, may have been left high and dry, for the face of the earth has not always been as we know it now.

In time all the little particles became welded together to form a new kind of rock. Sometimes animals and plants were buried in the mud destined to become a rock and their parts were so well preserved that they may not only be recognised by present-day scientists, but in many cases their structure may be made out so well with the aid of a microscope that no one would guess they had been buried thousands of years ago.

New rocks have been formed not only by the breaking up and welding together of the original earth’s crust, but by animal and plant remains. In some places, in past ages, billions and billions of little shell-fish have lived in the waters, died there and their shells have fallen to the bottom of their watery home. Now we know their shell remains as chalk or if it has undergone great pressure, owing to changes in the earth’s surface, as limestone. There are rock masses also formed of the remains of countless Diatoms; sponge spicules, too, have played their part in rock formation.

It is clear from what we have written that, in the first place, the rocks of the earth form two great divisions, the rocks which formed the original crust of the earth, Primary rocks they are called, and the later-formed Secondary rocks. The Primary rocks are often glassy in appearance, they show unmistakable signs of having once been molten and they never contain animal or plant remains, fossils as they are called, for the reason we have already explained. The secondary rocks often betray their origin by occurring in layers, or strata to speak more scientifically. Always when we think of the rocks we must think in thousands of years, then it will be easier to understand the formation of these strata, each one of which may represent the work of hundreds and hundreds of years.

Now to return to our sand; we have explained briefly that sand is really the remains of broken up rocks. First we have the solid rock, boulders are broken from it as the boulders are acted upon by rain, frost and wind: they become more and more broken up and at the same time they are carried towards the sea where they take the form of shingle. The shingle, in the course of time, becomes broken up into sand, and the sand again becomes so finely divided that it forms mud. From this it is clear that the composition of the sand depends largely on the nature of the rocks in the neighbourhood.

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.

For the most part the fossils of animal forms do not make good objects for the microscope. The Foraminifera, minute creatures dwelling in shells, are the most suitable for microscopic examination and very beautiful some of them are. They are best examined by reflected light, for then their little shells show their delicate pearly sheen.

Sponge spicules, as may be guessed from their hardness, are common in the fossil state; there are also fossils of sea-anemone ancestors and fossils also of Hydra colonies. The last named, known as graptolites, look not unlike lead-pencil marks on the rock. They exist in many forms; Diplograptus has a stem with two rows of cups in which the little creatures lived long ages ago; Monograptus has but one row of cups upon its stem, whilst Didymograptus is a branched form. These fossils are by no means rare and are worth studying under the microscope.

There are curious fossils knows as Trilobites, not unlike the present-day wood lice; they too may be studied, for some of them show all their characteristic markings as plainly as they must have appeared on the living animals.

In the plant world, many fossils in amazingly, good states of preservation have been found. Some of the giant Club Mosses from the coal measures, exhibit all the characteristic stem markings, leaf scars and the like, so clearly that one might imagine one examined a living specimen. Most of the plant remains are beyond the reach of the amateur, but many of them may be viewed in museums, in different parts of the country and the microscopist, whether he be a student of rocks or a lover of plants, should make a point of examining them. From quite fragmentary remains, scientists have been able to conjecture what vegetation covered the earth at various ages. The present is the age of flowering plants, but long ago the world was green with giant Club Mosses and Horsetails, very humble plants at the present time.