are called FOSSILIFEROUS; and the remaining rocks, from their containing no such relics, are called NON-FOSSILIFEROUS.

The changes produced by the united operations of aqueous and igneous agency are in part represented in the subjoined engraving of a supposed section of part of the earth's crust. Now to begin our lesson !

Here are three pieces of stone:-

I. A piece of Sandstone.
2. A piece of Granite.

3. A piece of Chalk.

You are quite familiar with each of these kinds of stone. Sandstone is a common material for walls, lintels, hearths, and flagstones. Granite may now be frequently seen in polished columns and slabs in public buildings, shops, and in tombstones; and the streets in many of our large cities and towns are now paved with it. Common white chalk is well known to everybody.

grains tend to lie in lines, and that these lines run in a general way parallel with each other.

5. The grains differ from each other in size and in the material of which they are made. Most of them consist of a very hard white or colorless substance like glass, some are perhaps small spangles of a material which glistens like silver, others are softer and of various colors. They lie touching each other in some sandstones; in others they are separated by a hard kind of cement which binds them all into a solid stone. It is this cement which usually colors the sandstone, since it is often red or yellow, and sometimes green, brown, purple, and even black.

Summing up these characters in a short definition, you might describe your sandstone as a stone composed of worn, rounded grains of various other stones arranged in layers. Proceed now in the same way with the piece of granite.

a

Take the piece of sandstone in your hands and examine it carefully, using even a magnifying glass if the grains are minute. Then write down each of the characters you observe one after another. You will of course pay little heed to the color, for sandstones, like books, may be red or white, green or yellow, or indeed of almost any color Nor will you give much weight to the hardness or softness as an essential character, for you may find even in a small piece of the stone that one part is quite hard while a neighboring place is soft and crumbling If your piece of sandstone has been well chosen for you, you will be able to write down the following characters :1. The stone is made up of small grains.

2. The grains are all more or less rounded or worn. 3. By scraping the surface of the stone these rounded grains can be separated from the stone, and when they lie in this loose state they are seen to be mere grains of sand.

You find at once a very different set of appearances, but after a little time you will be able to make out and to write down the following:

The stone contains no rounded grains.

It is composed of three different substances, each of which has a peculiar crystalline form. Thus, one of these, called feldspar, lies in long smooth-faced, sharply defined crystals of a pale flesh color, or dull white, which you can with some difficulty scratch with the point of a knife. Another, termed mica, lies in bright glistening plates, which you can easily scratch and split up into thin transparent leaves. If you com pare these shining plates with the little silvery spangles in the sandstone, you will see that they are the same material. The third, named quartz, is a very hard, clear, glassy substance on which your knife makes no impression, but which you may recognize as the same material out of which most of the grains

The crystals in granite do not occur in any definite order, but are scattered at random through the whole of the stone.

Here are characters strikingly different from those of the sandstone. You might make out of them such a short defini tou as this--Granite is a stone composed of distinct crystals, not laid down in layers, but irregularly interlaced with each other.

Lastly go through the same process of examination with your piece of chalk. At first sight this stone seems to have no distinct characters at all. It is a soft white, crumbling substance, soils your fingers when you touch it, and seems neither to have grains like the sandstone nor crystals like the granite. You will need to use a magnifying glass, or even perhaps a microscope, to see what the real nature of chalk is. Take a fine brush and rub off a little chalk into a glass of clear water, then shake the water gently and let it stand for a while until you see a layer of sediment on the bottom. off the water and place a little of this sediment upon a piece of glass, and look at it under the microscope or magnifying. glass You will find it to have strongly marked characters, which might be set down thus :

Pour

The stone though it seems to the eye much more uniform in its texture than either sandstone or granite, is made up of particles resembling each other in color and composition, but presenting a variety of forms.

It consists of minute shells pieces of coral, fragments of sponges, and white particles, which are evidently the brokendown remains of shells.

As a brief description of chalk you might say that it is a stone formed out of the remains of once living animals.

You should repeat this kind of examination again and again until you get quite familiar with the characters which have been written down here And you will see why it is im portant for you to do so when you come afterwards to find out that these three stones are examples of three great groups into which most of the rocks of the world may be arranged So that when you master the composition of a piece of sand stone, or chalk, or granite, and learn how each stone was formed, you not only do that, but lay a foundation of knowl edge which will enable you to understand how by far the greater part of the stones of our mountains, valleys, and seashores came into existence.

In spite then of the apparently infinite diversity of the stones of which the globe is built up, you see that by a little study they may be grouped into very few classes. You have to follow a simple principle of classification, and each stone you may meet with falls naturally into its own proper group You do not concern yourselves much with mere outer shape and hue but try to find out what the stone is made of, and ask whether it should be placed in the Sandstone group, or in the Granite group, or in the Chalk group.

WHAT STONES HAVE TO TELL US. We take again our three pieces of stone-sandstone, chalk, and granite--and compare other stones with them. We get out of town to the nearest pit or quarry or ravine, to any opening in fact, either natural or artificial, which will enable us to see down beneath the grass and the soil of the surface.

In one place we may find a clay-pit, in another a sandstone quarry, in another a railway cutting through chalk or limestone, in another a deep ravine in hard rocks with a stream flowing at its bottom. It does not matter for our present purpose what the nature of the opening be, provided it shows us what lies beneath the soil. In all such places we meet with stone of some kind, or of many different kinds. By a little practice we learn that these various sorts of stones may be usually arranged under one or other of the three divisions. For example, a large number of stones will be found answer. ing to the general description which you found to be true of sandstone. These will of course be placed together with our piece of sandstone. Another considerable quantity of stones will be met with made up wholly or almost wholly of the remains of plants or of animals These we arrange in the same division with our piece of chalk. Lastly, a good many stones may be met with built up of crystals of different kinds, and these, for the present we class together with our piece of granite

In this way you would advance from the mere pieces of stone which you can hold in your hand, up to the masses of stone lying under a whole parish or a county or even the entire kingdom

You would begin to perceive that the different kinds of stone are not scattered at random over the country, but have each their own places, with their own kinds of hills or valleys.

The solid earth under your feet has a history as well as the people who have lived on its surface. Take Great Britain for example. You will learn that once a great part of this country as well as of Europe and North America was buried under ice like Greenland Earlier still it had jungles of palms and other tropical plants, yet further back it lay beneath a wide deep ocean, and beyond that time can be traced many still more remote periods, when it was forest-covered land or wide marshy plains, or again buried under the great sea. Step by step you may follow this strange history backwards, and with as much certainty as you trace the doings of Julius Cæsar, or William the Conqueror.

SEDIMENTARY ROCKS.

I. What Sediment Is.

To each of these groups names must be given We might call them the Sandstone group the Chalk group, and the Granite group But it happens that other names have been already in use, which will be more convenient Accordingly we shall refer all stones having characters like those of sandstone to the Sedimentary Rocks; those formed of the remains of plants or animals, as chalk is, to the Organic Rocks; and those having a crystalline character, like our granite group, to the Igneous Rocks. The meaning of these names will be seen as we proceed.

The word "rock" is applied to any kind of natural stone, whatever may be its hardness or softness. In this sense, sand, mud, clay, peat, and coal are rocks, as much as sande stone, limestone, or granite

Sediment is something which, after having been suspended

in or moved along by water, has settled down upon the bot

tom.

The term Sedimentary Rocks is a very expressive one, for it includes stones formed of all kinds of sediment, whether coarse or fine.

II. How Gravel, Sand, and Mud are Made.

You have taken the first step in the study of the Sedimentary Rocks-you now know that they are made of sediment, such as gravel, sand, and mud. How then are gravel, sand, and mud made at the present day?

If you were to search on the shore of the sea, or on the banks of a river, you could, without much difficulty, prove in another way that sand and gravel only differ from each other in the size of their grains.

Let us get away up among the hills, and watch what goes on where the brooks first begin to flow Where the rocks are hard and tough, they rise out of the hill-sides, at prominent crags and cliffs, down which the little streamlets dance from ledge to ledge before they unite into larger streams in the bottom of the valleys. Now let us descend the brook and look at its channel carefully as we go The red fragments from that crag will be easily distinguishable from the other dull gray stones, which have been detached from the rest of the crags on either side. If you look narrowly at the bits of stone which are strewed about upon the slope you will notice that they are all more or less angular in shape, that is to say, they have sharp edges. But those in the brook are not quite so rough nor so sharp-edged as those on the bare hill-ide above. Follow the brook down the valley for some way and then take another look at the stones in the bed of the stream You do not now find so many big blocks of the red stone, and those you do meet with are more rounded and worn than they were near the crag. They have grown smooth and polished, their edges have been worn off, and many of them are well rounded. Once more you make a further examination still lower down the valley, and here and there where the stream has thrown up a bank of gravel you find that the pieces of our red crag have been so well ground away that they now form part of an ordinary water-worn gravel.

III.-How Gravel, Sand, and Mud Become Sedimentary Rocks.

So long as a current of water is moving swiftly it keeps the gravel, sand, and mud from settling down on the bottom. A rapid current will hurry along, not only mud and sand, but even gravel. As its rapidity flags, first the gravel will sink to the bottom as a sediment, the sand will sink more slowly and be carried farther, while the mud will hang in the water for a long time, travel a much greater distance, and only fall with extreme slowness to the bottom.

You can examine the bottom of a dried up pool and see exactly what took place when the muddy water filled it. Here at the upper end is the tongue of sand pushed out from the shore by the streamlet. You recognize it as a true delta. The bottom

of the rest of the pool is covered with fine muddy silt or sand spread out over all the space on which the water lay.

With a knife we carefully cut a hole or trench through these deposits on the floor so as to learn what they consist of from top to bottom A cutting of this kind is called a Section, and may be of any size. The steep side of a brook, the wall of a ravine, the side of a quarry or railway-cutting, a line of cliff, are all sections of the rocks Let us see what our section has

to tell.

Now

In the center of the little basin the sediment brought in by the rain has accumulated to a depth, let us say, of an inch, below which lies the ordinary surface of the roadway. what feature strikes you first about this deposit of sediment when you come to look at the section which we have cut through it? Are the materials arranged without any order? By no means The materials have been deposited in layers which have been laid down flat one above another. Some of these layers are finer, others coarser than the rest. But whether coarse or fine they all show the same general arrangement in level lines.

In this way you gradually would come to be convinced that one grand leading feature of the sedimentary deposits laid down under water is that they are not mere random heaps of rubbish but that they are assorted and spread over each other in regular layers This kind of arrangement is called Stratification, and the sediments so arranged are said to be strati fied. So characteristic is this mode of arrangement among the sedimentary rocks that they are often called also the Stratified Rocks.

The sheets of sand, gravel, or mud which can be seen on the sea-shore, or at any lake or pool on land, are soft or loose materials. Sandstone, conglomerate, shale, or any other sedimentary rock, is usually more or less hard or compact.

A sedimentary rock then is one formed from sediment which was derived from the waste of older rocks, and deposited in water. It usually shows the stratified arrangement characteristic of water-formed deposits. Since its original formation it has usually been hardened into stone by pressure or infiltration.

IV. How the Remains of Plants and Animals come to be Found in Sedimentary Rocks.

What is this black object lying on the upper surface of that stone? You see at once that it has the form of a plant and resembles some of the fern tribe. Examine it more closely, and tracing the delicate veining of the fronds, you cannot doubt that, although no longer soft and green, it was once a living fern. It has been changed into a black substance which, when you look carefully at it, proves to be a kind of coal. Little fragments and layers of the same black coaly substance may occur throughout the piece of shale. If you scrape a little off and put it upon the point of a knife, you find that you can burn away the black material while the grains of sand or clay remain behind. These fragments and layers are evidently only leaves and bits of different plants imbedded at the same time as the larger and better preserved

fern. Now how did plants find their way into the heart of a piece of stone?

Rain can wash away leaves and other pieces of plants, and allow them to drop in a pool, where they become interstratified with the silt, that is, are deposited between its layers and covered over by it.

You can now see therefore how it is that pieces of ferns or any other kind of land plants should be found in the heart of such a solid stone as our bit of shale. The stone was once merely so much sediment laid down below water, and the fragmentary plants were drifted away from the place where they grew until at last they were buried among that sediment.

It is not only plants, however, which occur imbedded in sedimentary rocks. You will notice a number of shells and other animal remains, chiefly trilobites, that is, little seacreatures belonging to the same great tribe with our common crab and lobster. You do not need now to be told how they came there. You have learnt that anything lying at the bottom of the sea or of a lake will be buried in sediment.

Any relic of a plant or animal imbedded in rock is called a Fossil.

V. A Quarry and its Lessons.

Let us suppose ourselves to be in a quarry.

In the first place what feature about the quarry strikes you most forcibly when you enter? You answer readily, the Stratification of the rocks. They are arranged in layers or beds, one above another, in that stratified arrangement which you have found to be so characteristic of rocks laid down as sediment under water.

In the second place, you observe that they do not all consist of the same materials Some are of fine conglomerate, others of various kinds of sandstone, and some of different sorts of shales or clays. These beds or strata as they are called, alternate irregularly with each other, just as gravel, sand, and mud might be found alternating in the delta of a river or under the sea.

In the third place, let us ask you to point out which are the oldest of the beds. You answer without hesitation that those at the bottom of the quarry must be the oldest, because they certainly were deposited before those lying above them. In all such cases the beds at the bottom are the oldest, and those at the top the newest This arrangement of one bed or stratum above another is called the Order of Superposition.

We split open some of the lower beds of sandstone and find their surfaces often covered with markings. If you have ever walked along a flat sandy beach you must have noticed the ripple-marks which the shallow rippling water leaves on the soft sand. They are precisely like those on the sandstone. You may see them too along the shelving margin of a lake, indeed whatever water has been thrown by the wind into little wavelets over a sandy bottom. They betoken shallow water. Hence we have learnt one important fact from our quarry, as to the origin of these rocks: viz., that they were not deposited in a deep sea but in shallow water.

We look still further among these strata, and notice at last that some of them are curiously covered with little round pits, about the size of peas or less. How did these markings come

there? You know that when drops of rain fall upon a smooth surface of moist sand, such as that of the beach, they each make a little dent on it.

Here then is another fact which throws still more light on the history of these rocks. The ripple-marks show that the water must have been shallow; the rain prints prove that it must have risen along a beach liable, now and then, to be laid dry to the air and rain. Now can we tell whether the water was salt or fresh ? in other words, was this beach the shore of a lake, or of the sea?

Again we turn to the rocks themselves, and from some of the layers of shale we pick out a number of fossils, which enable us to answer the question.

ORGANIC ROCKS, OR ROCKS FORMED OF THE REMAINS OF PLANTS AND ANIMALS.

I. Rocks Formed of the Remains of Plants. Since the leaves, branches, and stems of plants, and the shells or other remains of animals, are sometimes scattered so abundantly through ordinary sedimentary rocks, it is easy to see that sometimes they may occur in such quantity as to form great deposits of themselves. You could hardly call such deposits sedimentary, in the same sense in which common shale and sandstone are so named. We may term them Organic Rocks, or, Organically derived Rocks, because they owe their origin to the accumulation of what are called organic remains, or fossils, that is, the remains of plants or animals. A plant or animal lives, moves, and grows by means of what are called organs. For instance, we walk by using our legs, which are our organs of locomotion; we speak with our mouth, which contains our organs of speech; we see by means of eyes, which are our organs of sight; and so on. Every object, therefore, which possesses organs is said to be organized or to be an organism. So that when you see this word organism you will remember that it means either a plant or an animal, for it is only plants and animals which are really organized

We begin with those rocks which have been formed out of the remains of plants. As an illustration let us ask you to examine carefully a piece of coal. If you master all that it has to tell you, you will not have much difficulty in tracing out the history of other rocks belonging to this series,

Now look at one end of a lump of coal, where the edges of the layers are exposed. You cannot follow them with the same ease as in the case of a piece of shale, for they seem to blend into one another. But you may notice that among the layers of hard, bright, glossy substance, there occur others of a soft material like charcoal. A mere general look at such a piece of coal would show you that it is stratified.

You know that coal can be burnt away so as to leave only ashes behind, and that in this respect it resembles wood and peat. Chemists have analyzed coal and found that it consists of the same materials as wood or peat, and that in reality it is only so much vegetation which has been pressed together, and gradually changed into the black substance now used as fuel. Let us suppose ourselves at a coal mine. Now, first of all, you see that the coal occurs as a bed, having a thickness of a

few feet. This bedded character agrees with what you have already noticed as to the internal layers in the stone, and confirms you in believing that coal is a stratified rock. Next ob serve that the pavement on which the coal rests, and the roof which covers it, are both made of very different materials from the coal itself. Were you to cut a trench or section through pavement, coal, and roof, you would prove beyond any doubt that the bed of coal lies among beds of common sedimentary rocks.

You are driven to conclude that in truth the under-clay is an old soil, and the bed of coal represents the vegetation which grew upon it.

II. Rocks Formed out of the Remains of Animals. It is on the floor of the great sea that the most wonderful examples occur of the way in which rocks are gradually built up from the remains of animals to a depth of many hundreds or thousands of feet, and over distances of many hundreds of miles.

To the west of Britain the Atlantic soon and suddenly deepens. Its floor then stretches away to Newfoundland as a vast plain, the lowest part of which is about 14,000 feet below the waves. It was over this wide submarine plain that the telegraph cables had to be laid, and hence numerous sound. ings were made all the way across from Ireland to the American coast. While in the shallower parts of the sea the bottom was found to be covered with sand, gravel, or mud, from the deeper parts there came up with the sounding-lead a peculiar gray sticky substance known as ooze, which must stretch over that wide deep-sea basin for many thousands of square miles. This ooze when dried looks like a dirty kind of chalk. After the lapse of centuries, if the deposit were to remain undis turbed, and if we could set a watch to measure its growth, we should find it to have risen upward and to have inclosed the remains of any star-fishes or other sea-creatures which chanced to die and leave their remains upon the bottom. Hundreds of feet of such slow-formed deposit have no doubt already been laid down over the bottom of the oceán between Ireland and Newfoundland. Here then is a second and notable example of how a deep and far-spread mass of rock may be formed out of the remains of animals.

IGNEOUS ROCKS.

I. What Igneous Rocks Are.

This word igneous means literally fiery. It does not very accurately describe the rocks to which it is applied, but it has long been in use to include all rocks which have been actually melted within the earth, or wich have been thrown out at the surface by the action of volcanoes. So that the Igneous Rocks owe their origin to some of the effects of the internal heat of the earth.

You will find that the solid materials cast up by volcanoes are of two kinds-Ist, streams of molten rock called Lava, poured down the sides of a volcanic mountain during an eruption: and 2d, immense quantities of dust, sand, and stones, cast up into the air from the mouth of the volcano, and falling down upon the mountain, sometimes even all over the sursounding country for a distance of many miles.

Here then are two very dissimilar kinds of rock-material discharged from the interior of the globe. The lava cools and hardens into a solid rock. The loose ashes and stones, likewise, are in time pressed and hardened into more or less firm beds of stone. So that two totally distinct kinds of rock are laid down upon the surface of the earth by the volcano. In the case of the lava, the rock, if you look at it with a magnifying glass, is seen to be made up of distinct crystals all matted together. The beds of ashes, on the other hand, no matter how compact they may have become, are found to be made up of irregular fragments of various kinds of stone, and of all sizes, from the finest dust up to big blocks. By attending to this very simple and intelligible difference you could arrange igneous rocks into two great groups-Ist, the Crystalline, that is, those which are made up of crystals, and which have once been in a melted state; and 2d, the Fragmental, that is, those which consist of the loose materials thrown out during volcanic explosions.

Crystalline Igneous Rocks.-When the rock was still melted it was full of imprisoned steam and gas which were constantly striving to escape to the surface. It was this steam which collected into little bubbles and formed the curious set of holes in the mass of the still molten rock. In the same way the holes which you often see in the heart of a loaf of bread were formed by the struggles of the steam to escape from the dough as it was heated in the oven.

There are Fragmental Igneous Rocks.-Now this is the kind of material under which the old Roman city of Pompeii was buried. It fell upon the streets and houses and gradually covered them up as the eruption of the neighboring volcano continued. And at this day the workmen find the streets and chambers all choked up with layers of coarser and finer volcanic ash and dust.

These masses of consolidated volcanic dust and stones are known by the name of Tuff.

II. Where Igneous Rocks Come From.

If we ask you from what source the Igneous Rocks have been derived, you will reply that they have come up from the intensely hot regions within the earth.

Deep Borings and Mines.-If you were taken down to the bottom of a deep mine in the United States, you would find the temperature much warmer there than near the sur. face, and a similar increase of heat would meet you in the deep mines of every country in the world. You would soon discover, too, that on the whole the deeper the mine the greater the warmth would be. In the same way were you to bore a deep narrow hole into the earth for several hundreds of feet and let a thermometer down to the bottom, you would find that the mercury would rise in the tube.

Experiments of this kind have been made all over the globe, with the result of showing that after we get down for a short and variable distance below the surface, we reach a temperature which remains the same all the year, and that underneath that limit the temperature rises about 1° Fahrenheit for every fifty or sixty feet of descent. If this rate of increase continues, we should get uncomfortably hot before having descended very far. For instance, at a depth of about two miles water would