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(combinations of an element with oxygen and silicon, or carbon or sulphur respectively). Coal is nearly pure carbon; some of the metals are not rare native or combined with sulphur, and rock salt is the gas chlorine combined with the metal sodium.

The composition of the earth's interior, and of the crust which was formed when the mass began to solidify, can only be inferred from examining those rocks which have been and are from time to time ejected from within. These are all silicates, and as they have issued forth in a molten state are called Igneous rocks. All other rocks found upon the face of the earth must have been formed directly or indirectly from these; directly when they are made up of broken (and generally water-worn) fragments; indirectly, when composed of minerals or of the solid parts of plants or animals, the materials of which must have originally come from the same source.

As water has played a large part in arranging the materals in the latter class, rocks belonging to it are called | Aqueous; and as their materials are usually arranged in tolerably regular strata or layers, these are called the stratified rocks, in contrast with the unstratified or Igneous rocks. A third division is usually made, containing rocks which, though stratified, have since their deposition been much changed (chiefly by heat, pressure and water); these are on this account called the Metamorphic or altered rocks. We must now notice one or two facts of rock structure which are of the highest importance. It was said that the Aqueous rocks were arranged in more or less regular layers or strata. These you may notice for yourself in any district where clear faces of rock are exposed in quarries, or in gullies cut by streams, or in cliffs on the shore. Sometimes they are so regular as to resemble courses of masonry. Sometimes, as in the chalk, there is one great nearly uniform mass several hundred feet thick. Change from one stratum to another, as you will presently see, signifies some change in the materials that were being deposited.

Not seldom, in examining a stratum, we find it made up of thin layers. These are called lamina. Place several books one above another, resting on their sides. Each book represents a stratum, its pages the lamina which make up the stratum. Sometimes, however, the laminæ do not run thus evenly, but slope or curve downwards from top to bottom of the stratum. This is called False or Current Bedding; the latter name being given because (as you can learn from almost any river bed) it is caused by the action of streams.

Rocks that are at all laminated will obviously split most easily along these layers; thus paving flags are obtained We also find rocks which split readily, but in quite another direction. Changes in the color or nature of the materials show us how the rock was deposited, but the lamina now stick fast together, and the rocks split without any regard to them. This is called cleavage, and rocks which so split are properly called slates. Of the cause we will speak hereafter. In other altered rocks not only is there a tendency to split, not necessarily brought about by the original bedding, but also the materials of which the rock is made up appear to have been more or less altered. As these rocks often have a rough resemblance to layers of flattened leaves, they are said to be foliated; and as they split are called schists, from a Greek word which has this meaning.

These structures* are confined to rocks that once might have been classed as Aqueous. We next come to an important structure which equally affects Igneous, which we find more or less distinctly present in all but the softest and most incoherent rocks. You can not walk for a few

*A rude cleavage has sometimes been noticed in Igneous rocks From the explanation of cleavage which follows the student will see that this might be expected.

minutes in a quarry or by the side of a cliff, without noticing that the rock is cut up into blocks, sometimes of very regular shape, by a number of clefts. When you look more closely at these you will find it easy to arrange them in sets, each running with remarkable evenness in some particular direction. There are usually two or else three such sets, and two at least of them are not far from being at right angles to the top and bottom of the bed. These divisions are called joints-of course they are great helps to the quarrymen, who often call the most important of them master joints. I have seen them sometimes in the South Tyrol Alps, extending with almost unbroken regularity down the faces of cliffs many hundreds of feet in height. The rock in this case was very uniformly stratified, hence the evenness of the joints; for, as a rule, they change when the nature of the beds alters. Joints may be seen in a quarry of Igneous rock, near North Queensferry (Fife), breaking the mass into great blocks at least fifty or sixty feet long, whose sides are as smooth as a plastered wall.

In conclusion, these structures, Cleavage, Foliation, and Joints, demand a few words of explanation; although we can not attempt to do this in more than the roughest way, seeing that the subjects are difficult and there is much that yet remains somewhat uncertain. 24

(1) Cleavage.-The cast-iron rails on a railway after a time flake off; bees-wax when compressed becomes cleavable. It is observed that fossils and lumps of clay included in a slate always appear to have been crushed by a force which has acted at right angles to the direction of cleavage; and in some cases, where coarser bands exist to show us more distinctly what has happened, these are crumpled up in a way that could only be done by a force acting as above. It seems then reasonable to conclude that cleavage is the result of pressure, and was probably caused by some of the great movements which the earth has undergone. As might be expected, it is usually found in the older rocks.

(2) Foliation-Here there is a chemical as well a mechanical change. The grains forming the original sand, mud, etc., can no longer be seen, at any rate in their former conditions; the rock is quite altered, and new minerals are present in it. Heat and water have probably caused this change. We can not here, as in the last, imitate Nature's experiments, but by examining rocks near those that have been melted, or near warm mineral springs, we are led to believe, as you will some day learn for yourselves, that these agents have brought about the change. The foliated structure appears to depend to some extent on the original bedding of the rock, and to some extent on subsequent pressure.

Heat and water have also, no doubt, converted sandstones into quartzites, clay into hornstone, and common into crystalline limestones. Whenever these changes have taken place on a large scale it is probable that the operation has occupied a long time, and that the rock has been, as it were, slowly "stewed." Though simple "baking" produces similar effects, as may be seen near intrusive masses of lava, these, as a rule, do not appear to extend far into the rock.

(3) Joints.-The cause of these is probably shrinking, just as the cracks are formed in the mud of a dried up pond. Their regularity over large surfaces is probably produced by this having taken place slowly and uniformly. This shrinking may take place either in cooling or in drying. In igneous rocks the joints will be produced in the former way, in aqueous in the latter. The most remarkable case of jointing is to be seen in certain igneous rocks, where the whole mass is divided into regular columns, usually six-sided. This occurs most commonly in basalt, but it is not confined to it. Good examples of these columns may be found at the Giant's Causeway, in Staffa, and at Pouk Hill Quarry near Walsall. Often the columns run straight from the top to the

bottom of the bed, but occasionally, as in the Clam Shell - Cave, Staffa, and at Pouk Hill, they are curiously bent.

We consider that these columns are due to cooling, for the following among other reasons: The columns (when not bent) are always at right angles to the surface of the lava. If it has cooled on a plain, they are upright, if on the slope of a hill, they are no longer so; in a vertical dyke, they lie flat; and sometimes, as they began at the same time from the two sides, their ends, as might be expected, do not fit in the middle.

How is it then that the columns are generally six-sided? When a mass of molten rock is cooling, this appears to begin on the surface around a number of centres at about equal distances from each other; toward these the particles of the rock will shrink. Now, take seven coins of the same size, put one on the table, and you will find that the other six can be placed in a ring round, so that each touches it and two of the others. Suppose A be the central coin, B and c two of the others, D, E, F, the points where they touch. Draw straight lines to touch each pair through these points. It is known that they will meet in one point G. Now it is clear that when the rock is shrinking uniformly round and toward the centres A, B, C, all parts within the lines E G, G D, will shrink toward A, within G D, F G toward B, and within E G, F G toward c. Thus cracks will be formed along the lines E G, G D, F G. Now the cracks E G, G D, will define one angle of a column, with A for its middle point; and thus by passing on to the other circles we shall obtain a six-sided column formed about A; and in like manner we should get similar columns formed about the other centres. If the cooling goes on uniformly, the cracks thus begun will go on descending into the mass, and a set of perfectly regular columns be formed. The bent columns must be caused by some irregularities in the cooling, though it is not easy to say precisely what in each case. Some bendings also may be explained by a movement of the lava stream after it has partly cooled, but before it has become quite solid. Again, the columns are often seen to be divided across, as if they were built of separate stones, and when parted they show a ball-and-socket structure. Sometimes this is so marked

that they resemble a pile of Dutch cheeses. This must be caused by the cooling not going on quite uniformly in the mass. Thus though the direction of the column is kept up, another ball is started just beneath the one that is above, and the column is accordingly cracked across at this place. This explanation of the cause of joints-viz., contraction, whether from cooling or drying-is confirmed by the fact that miniature columns are found to have been formed in mud, coal, and other rocks, over which streams of molten lava have passed, and in the clay with which blast-furnaces are lined, as well as in starch and several other substances that have become solid in drying.

CHAPTER II.

Now that we have mentioned the general facts of structure in rocks, we must proceed to examine a little more minutely into their composition. The number of elements which make up the greater part of rock masses is small, as was said above; the number of minerals, or compounds of these elements, is also not very large. The following descriptions may help you to recognize the commoner of these, without any knowledge of the science of mineralogy, though the sooner you can learn something of this the better.

A large number of minerals are found to assume certain definite forms, which can be classified. These are called crystals, and are a great help in determining an unknown mineral. Again, in most cases it is found that the crystal will split in certain directions more easily than in others.

Get some crystallized barley sugar from the shop, and try to split it; this, you will find, can be done when the knife is placed so as to cut off a regular pyramid from any corner. This property in minerals of splitting more easily in some directions than in others is called cleavage, and it is an important help in recognizing them. You must be careful not to confuse mineral cleavage with rock cleavage, for the cause of them is not the same. Mineral cleavage (i. c., the cleavage of crystals) is due to the mode in which the crystal is built up; the molecules which have formed it being laid in courses, something like the bricks in a wall. Rock cleavage is produced by changes after the mass is formed; as if (to give a very rude example) we could turn the bricks in a wall after it was built, so that the courses should run from top to bottom instead of in the usual manner. We have also to notice the hardness of a specimen, its color when whole and when powdered, and what effect acids have upon it. Taste and smell are sometimes used in distinguishing minerals, and of course there are other less rough tests which can be employed by the chemist in his laboratory.

If, then, you wish to recognize minerals or rocks, you must get a hammer, a pocket-knife, and a small bottle of hydrochloric acid.* Many of the common minerals and rocks can be identified from descriptions by the help of these, though of course you will find it a great advantage if you can get access to a collection in which the specimens

are named.

In the following list of the commoner minerals, we give, first, the minerals consisting of a single element; next, the compounds of two elements, then those of more than two.

ELEMENTS.

Carbon.-The diamond is pure carbon; so is graphite, plumbago, or "black-lead." The latter may be known from

the metal lead by its being much lighter and marking paper more easily. As a rule, however, small quantities of other there are valuable mines in Cumberland. The name is deminerals are present in the graphite of commerce. Of this rived from a Greek word, meaning "I write." Coal is nearly pure carbon; those varieties which soil the fingers least are the purest.

Sulphur.-Usually pale primrose yellow; can be recognized by the odor. Not common, especially in Great Britain.

OXIDES, ETC.

Silica.-One of the most abundant minerals in the earth's crust. The crystals are generally six-sided towers with sixsided pyramidal roofs. Properly it is perfectly transparent, when it goes by the name of rock crystal, and is largely used for lenses and ornaments, but it is often tinted with some mineral: as purple, when it is called amethyst; brown, cairngorm; yellow, false topaz, and so on. It occurs also in masses, not distinctly crystallized, when it is commonly called quartz; these are generally more or less opaque, and often dead white in color. Chalcedony is a milky; carnelian a reddish kind; jasper an impure and richly colored variety; agates are mixtures of these. Quartz generally occurs in rocks, either in veins or in small grains, of a greyish or whitish color. A bit of it cannot be split; hydrochloric acid has no effect on it. It is very hard; you cannot scratch it with a knife; the most that you will do is to spoil the point, finely crystalline a state that to the eye it looks quite comand make a bluish mark on the quartz. Flint is silica in so pact. Common opal is uncrystallized silica; it is not quite

so hard or so heavy as the same mineral when crystallized,

*The hammer and knife will tell us whether the mineral cleaves easily or not, the knife whether it is hard or soft; if a little of the acid causes a bubbling, or “fizzing" up—effervescence—you may know that the mineral contains carbonic acid gas, and thus is a carbonate.

and is of a dull, white color; the valuable gems, noble opal, with its delicate rainbow tints, and fire opal, with its rich orange lustre, are rare varieties of this form of silica. This mineral (especially of the uncrystallized kind) is dissolved in small quantities in most natural waters, particularly those of warm springs, which often deposit it as "sinter;" it is also present in certain vegetable tissues, especially in grass, straw and bamboo stems. It also forms the covering of certain minute animals.

Hæmatite.-One of the ores of iron. Among other places largely found in Lancashire. Not difficult to recognize; purplish or reddish brown in color, often making a reddish mark on paper, generally occurring in lumpy masses, heavy. Common iron-rust is oxide of iron combined with water. Rock-salt (Chloride of sodium).-More or less transparent, sometimes yellowish or reddish. The purer kinds can be split in certain directions;, it is quickly dissolved by water; easily known by taste; can be scratched without difficulty;

does not effervesce.

Fluor-spar (a combination of the elements fluorine and calcium) is a not uncommon mineral; it is often nearly transparent. Crystals cube-shaped, clear, the color yellowish, greenish and light to dark purple. Can be scratched without difficulty; does not effervesce. The Blue John of the Derbyshire spar-shops is this mineral.

SILICATES.

The commoner of these are composed of several elements. The following are most frequently met with:

Serpentine.-A silicate of magnesia chemically combined with water, and usually tinted by other minerals; dark mottled red and green being the commonest colors. It does not form crystals, so a freshly broken surface does not glitter; can be rather easily scratched with a knife. All minerals containing much silicate of magnesia have a soapy feeling when handled.

Talc (not the mineral often sold under that name) has less water in its composition, is of a pale greenish color, with a pearly surface. Can be easily scratched with the finger-nail.

Augite and Hornblende are two minerals very similar in all respects, except that the crystals differ somewhat in form. Generally they are dark green to nearly black in color, and rocks which contain them are usually dark colored, and, as much iron enters into their composition, assume a rusty brown color when exposed to the weather. It is often very difficult to distinguish these minerals; hornblende crystals, however, not unfrequently resemble bundles of needles or fibres of wood, and one variety, asbestus, is like flossy silk; this is used for gas-stoves. The little scales or plates of a bronze-like or greenish mineral, common in serpentine, are a variety of augite.

Felspar.-A very important group of minerals. They all consist of silicates of alumina, together with silicates either of potash, or soda, or lime, or iron, etc., the latter producing the different varieties into which the group is divided. Felspar usually occurs in rather long crystals, which split in two directions, one rather more easily than the other. They are generally opaque, with a satin-like sheen on the split faces. Color dead white, flesh-tint, or dull green; can be scratched, though with difficulty, with a knife; the acid will not affect them. In potash-felspar the cleavage faces are at right angles. This mineral, often called orthoclase (right-angle-cleaving), is abundant in granite, in which it sometimes forms large separate crystals. The other felspars (with the exception of one rare variety) do not cleave at right angles. They may be recognized by this and by the presence of fine parallel lines on some of their cleavage faces. It is often not easy to distinguish the different varieties. One called Labradorite, from the place where it is

especially found, often shows a beautiful play of colors, blue and green, as on a peacock's feather.

Mica. This name also covers a group of minerals of rather varying composition. All, however, are distinguished by splitting very readily in one direction, so that they can be flaked off with the point of the knife. Color various, from silvery white to black, with a metallic lustre. Can be scratched more easily than felspar. Not immediately affected by acid. The plates of one kind are sometimes transparent and large enough to be used in commerce for covers to written labels, for smoke protectors above gaslights, etc. This is the mineral popularly called talc.

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Calcspar (Carbonate of lime).-This mineral is very abundant. The crystals vary much in shape, but commonly are sharp pyramids, often transparent, sometimes colored. You may distinguish them from quartz by the following tests: They split or cleave in three directions, forming somewhat oblong (but not right-angled) blocks. They can be scratched without difficulty with the knife. When acid is dropped on them a bubbling-up takes place. Carbonate of lime is the principal ingredient in chalk and limestones, corals, shells, and in bones, and the solid parts of most animals. Dog-tooth spar, nail-head spar, Iceland spar, are names given to some varieties, of which the first is commonest.

SULPHATES.

Gypsum (Sulphate of lime).- Crystals rather oblongshaped, somewhat pearly in appearance, softer than calespar (the thumb-nail is just able to mark it), and splitting not with equal ease in three directions; often found in clays. The rock gypsum consists of this mineral in an uncrystallized, or very finely-crystallized state. The natural color is snowy white. Gypsum can be distinguished from limestone by its greater softness and by not bubbling up when touched by the acid.

With the exception of iron, none of the "metals" enter generally, like the above minerals, into the composition of large rock masses, but they occur in various hollows, cracks, and veins, or as scattered crystals, singly or in groups, in them. Iron occurs, as has been said, as the oxide. Another common form is iron pyrites (a combination of sulphur and iron-sulphide of iron); the crystals are cube-shaped, can not be marked with the knife, and resemble brass. The carbonate of iron occurs in "clay ironstone," and in what are called chalybeate springs. The oxide of copper may be known by its resemblance to copper. Copper pyrites is more copper-colored and softer than iron pyrites; the carbonate is a bright green or a deep blue; one variety of this is the malachite, for which Siberia is so celebrated; the commonest ore of lead is the sulphide: this may be known by its weight and general resemblance to the metal. Gold is often found pure, silver not seldom; the ores of it and of tin can not be briefly described or very easily recognized.

We next proceed to the rocks which are composed of the minerals described above, namely, the Igneous, Aqueous, and Metamorphic.

(1) IGNEOUS.-These are all composed of silicates, and sometimes contain silica as well, the crystals being generally rather small and often imperfectly formed. A crystal is like a tree, it must have plenty of room to grow well. Igneous rocks are often very difficult to classify, and the beginner must not expect to be taught much about them. Usually they are divided into two classes, according to the circumstances under which they have become solid. Those which have cooled down on or very near the surface are called Volcanie; those which have cooled down at some depth are called Plutonic. It is evident that no very hard and fast line can

be drawn between these; for if an igneous rock could be traced downwards, that which was at the surface Volcanic would be likely to become at last Plutonic. Some writers make an intermediate class to include those which have cooled down under moderate pressure; calling them Traps, from a word meaning a stair, because they often form steplike hills; but I do not think anything is gained by this division, except that the term is useful when a more definite one cannot be employed.

VOLCANIC ROCKS (POPULARLY CALLED LAVAS). The following are the principal varieties:Trachyte.-This rock is mainly composed of potash-felspar, generally grey in color, rough in the hand, rather light. Sometimes it becomes glassy and passes into Obsidian or volcanic glass—a rock very like ordinary wine-bottle glass. Sometimes it is slaggy, and when full of holes it is called Pumice. It is a common rock which has not yet been recognized in Britain.

the latter kind. Granite from Cornwall is used in Waterloo Bridge, from Devonshire in London Bridge (both greycolored), from the Channel Isles in London streets-these vary in tint. Peterhead granite is reddish; Ross of Mull, pinkish; Aberdeen greyish in color. Ireland also has several fine granites, which, however, are not often brought to England.

Frequently the bubble-like cavities in the volcanic rocks are filled up by minerals that are deposited there by water (such as calespar or chalcedony). These are called amygdaloids, from a Greek word meaning an almond, because the rock then somewhat resembles a slice of almond-toffy. Good examples occur in the Derbyshire Toadstone (Basalt). In the Igneous rocks especially certain minerals are very discrystallized or has a compact appearance, like hard cheese or tinctly crystallized, while the mass of the rock is either finely a broken face of porcelain. It is then said to be Porphyritic, and rocks of the latter kind are sometimes called Porphyries. This term also comes from a Greek word, which means a

Basalt.—Mainly composed of another variety of felspar red-purple, and was given to a valuable Egyptian rock, and of augite; crystals so small as not to be distinguished by the eye; purple black in color, often rusty brown when exposed to weather; heavier and much less commonly full of holes than trachyte. It contains more or less iron oxide. When more coarsely crystallized it is called Dolerite. It also is often slaggy, and its glass (rare) is called Tachylite. Basalt occurs at Rowley Regis and near Walsall, Staffordshire, where it is largely quarried for road metal, etc., also (including Dolerite) in other parts of England, especially in the north, and is enormously abundant in the central valley of Scotland, the inner Hebrides, and North of Ireland.

PLUTONIC ROCKS.

Felstone has much the same composition as, but is more compact and stony than, most Trachyte; being, no dou'st, the same rock more slowly cooled. It varies a good deal in color, some kinds being cream-white, others grey or greygreen, others reddish or purplish. In some grains of quartz are common, in others the felspar crystals are easily seen with the eye. Parts exposed to the weather are often (but not always) lighter colored than the rest. Its glassy state is called Pitchstone, which differs from Obsidian in being duller and more variable in color and rather more like resin in appearance. Felstone is abundant in North Wales, as at Penmaen-mawr, where it is largely quarried, the English Lake Country, and in many parts of Scotland. Its varieties are numerous. Pitchstone is much less common, being confined to a few localities on the west coast of Scotland and in Ireland.

Greenstone.-Several somewhat different rocks must be included under this head; the most familiar consists of a felspar (not potash-felspar) and hornblende, which is properly called Diorite. The rock is generally a dullish green in color, or speckled white and green, and brownish when weather-worn. It is often difficult to distinguish it from Dolerite.

Syenite.-A compound of potash-felspar and hornblende, generally speckled dull green and pink or white. There are extensive quarries of it in the Malvern Hills and at Charnwood Forest, and it is found in some other parts of Britain. It, however, has been doubted whether, at the above localities, it is really of igneous origin. Most of that at Malvern is a metamorphic rock.

Granite. This important rock is a compound of quartz, potash felspar, and mica. The first of these minerals generally occurs in rather irregular grains, whitish, greyish or blackish in color; the second is distinctly crystallized, white, yellowish or pink red; the third is very variable in quantity and kind; usually it is either silvery-grey or metallic black. Honblende sometimes replaces mien. The celebrated quarries of Mount Sorrel (Leicestershire) furnish

called now rosso antico by marble workers, which was of that color, and was relieved by small distinct whitish crystals. It is one of the many varieties which we have been obliged to include under the single name Felstone. Some of the Granites, as those from Devon, Cornwall and Shapfell are porphyritic, and the potash-felspar crystals are often a couple of inches long. There are plenty of instances to be seen in London, the streets of which are becoming quite a museum of rocks on a large scale. A good example will be found on the approach to London Bridge.

(2) AQUEOUS.-We come next to the second great division of rocks, and here it may be as well to state that in geological language a rock is not necessarily hard; loose sand, soft mud, and firm stone are equally spoken of as rocks. The aqueous rocks are made up of fragments, sometimes quite large, sometimes so very small as to be hardly visible without a magnifying glass, which at some time or other have been supplied by the Igneous rocks. As might be expected the different varieties pass into one another even more than the former class. When Nature makes dirt-pies she mixes the ingredients without any fixed recipe. The following, however, are the chief varieties:

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Any rock which is principally made up of broken, sharpedged fragments of others, cemented together by some kind of mineral paste, is called a Breccia. This is imitated in some of the "artificial stones" used for pavements. When the fragments are rounded into pebbles, we call the rock a Gravel if they are loose; a Conglomerate or Pudding-stone if they are cemented together. 'Concrete," used in the foundations of buildings, is an artificial conglomerate. If they are mere grains (usually of quartz), and coarse, it is called a Grit; if fine, a Sand-stone; and if not cemented together this is Sand. Grits and sandstones may be rendered solid by mere pressure; but usually the grains are cemented together by a little hardened clay, or carbonate of lime, or iron rust, or even silica.

When rocks containing a considerable quantity of the mineral felspar are weathered or water-worn, some parts of it are dissolved, and the remainder consists of a chemical combination of silica and alumina-a silicate of alumina. This forms Clay. The purest kinds are used in making pottery: those mixed to some extent with fine sand or other minerals are the brick earths or clays. Rather hard laminated clays are called Shales, and those which contain some carbonate of lime are Marls. These rocks are rarely very hard, unless they have been to some extent baked by the action of Igneous rocks, and then they belong more properly to the next division.

Loun is clay mixed with fine sand; ordinary earth or soit is loam with some carbon from decayed vegetable matter..

Flint, as we have already said, is nearly pure silica. The mode in which it is formed will be described hereafter. When mixed up with some lime or even clay it is called Chert.

Limestones, as it has been stated, are composed of carbonate of lime. Chalk is a rather soft and uncrystallized variety; those which are crystallized, hard, and capable of being polished, are Marbles. Usually some clay or fine sand is mixed up in a limestone. One which may be described as a clayey chalk is called Clunch. Thus soft limestones pass through clunch into marl, and this by losing the lime passes into clay; hard limestones become cherty, or pass into calcareous sand-stones, and these, or clays, may get more and more sandy, till at last they become ordinary sandstones. Many lime-stones are burnt for quicklime (the heat driving off the car'bonic acid gas and leaving behind the oxide of lime). This is used in dressing land and in making mortar. Some also afford valuable building stones, as, for example, the quarries of Portland, Bath, Cheltenham, Ketton, and Ancaster. The first is used in St. Paul's Cathedral and in many of the City churches. Dolomite is a combination of carbonate of lime and carbonate of magnesia. There is a considerable quantity of it exposed in a strip of country between Nottingham and South Shields. York Cathedral, the New Houses of Parliament, and the Museum of Geology in Jermyn Street, London, are built of Dolomite or Magnesian Limestone, as 'it is often called. It is a handsome stone, but not always "very durable.

Coal, the origin of which will be described hereafter, is nearly pure carbon; shales and limestone also are often carbonaceous or bituminous, from the presence of decayed vegetable or animal remains. There is sometimes so much of this bitumen in shale that gas can be made, or mineral oils -such as paraffin-distilled from it.

It may seem, from what has been said, a difficult matter to classify these aqueous rocks; but the general character of a specimen can usually be easily determined in practice. A magnifying glass will show the grains of sand, if much of that is present-and a flinty or cherty rock may be known by its being hard to break with a hammer, by its "flying" into sharp-edged pieces when it does break, and by its being difficult to mark with a knife. Rocks with much clay in their composition, though not always easily broken, are readily bruised. They have also a peculiar earthy smell when damp. A little hydrochloric acid dropped upon a rock when powdered will show at once by effervescence whether there is any considerable quantity of carbonate of lime present; and the carbonaceous or bituminous rocks are dark colored, blaze and glow when put into the fire, and, if rich in bitumen, have the peculiar odor of this mineral.

Carbonate of lime, silica, and some other minerals, not unfrequently, when present in rocks, make their way to certain points in the mass, where a rather larger quantity of the mineral happens to be present, or where they can be readily deposited, and form what are called concretions. The hard globular masses called "ball stones," "corn stones," "septaria stones," are thus formed, which appear to have collected together almost all of some particular mineral (commonly carbonate of lime) from the surrounding rock. Septaria stones have cracked, probably in drying, and the cracks are filled up with pure carbonate of lime, and occasionally with silica. The lumps, rich in phosphate of lime,* which are now largely worked for artificial manures in the eastern counties of England, are probably concretions. The limestone called oölite (oon, egg; lithos, stone) or roe-stone, from its resemblance to the roe of a fish, is made up of a

*Called Coprolites, because it was at first supposed that they were fossil excreta: from kopros, dung, lithos, stone.

number of very small concretions, the centre of each of which is a minute shell or grain of sand. Sometimes these concretions are about as large as peas, then the rock is called pisolite (pea-stone). A very remarkable bed of this occurs in the Lower Oölite, near Cheltenham.

A clay

(3) METAMORPHIC ROCKS.-This class remains. or shale, which has become hardened, is called a hornstone, and when cleavable a slate; a sandstone solidified so that the edges of the grains look as if they had been melted together-like those in a sago pudding—is called a quartzite. Of the foliated varieties, gneiss consists of quartz, felspar, and mica, mica schist of quartz and mica. There are also hornblende schists, tale schists, chlorite schists (generally of a greenish color), containing the minerals after which they are named. Serpentine is believed to be a metamorphic rock, sometimes of igneous, sometimes of aqueous origin.

With this too brief account of the principal kinds of rocks we must content ourselves. Of these the earth's crust is made. You must not, however, suppose that they are disposed regularly one above another like the coats of an onion. What the first crust may have been like we can only conjecture, or how far the "veneering" may have at first been done with some regularity. All we can now say is that the surface was a "knubbly" one when it began to be patched over with the existing deposits, and that we have nowhere found the bottom rock in its original condition. Those pictures, which you see in some old books, of granite as the floor, gneiss and schist upon that, slate upon them, and the various stratified rocks on it, give no correct idea of the real state of the case. There are schists in the Alps which were deposited at about the same time as some clays in the Isle of Wight, and there are clays and sands near St. Petersburg of about the same age as some of our Welsh slates. In one country you will find a quite modern rock resting upon a very ancient one; in another you will have an interval of many thousand feet between the representatives of these two, filled up with all or many of the missing deposits.

[End of required reading for October.]

[blocks in formation]
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