world, forming a considerable part of many mountains, being often the principal stratum to a great depth below the surface. Part of the summit of Mont Perdu, the highest of the Pyrenees, is composed of secondary limestone arranged in nearly vertical strata, and so full of the remains of marine animals, as, in some places, to appear as if consisting of nothing else. The base of Ingleborough in Yorkshire, nearly thirty miles in circuit, consists entirely of limestone, containing vast quantities of sea-shells. Limestone forms the principal inferior strata through the greatest part of Derbyshire, arranged in beds of various thickness, from a few inches to two hundred fathoms, and abounding with shells and other marine remains. This substance is found in many parts of Scotland distinctly stratified.

Secondary limestone often contains metallic veins; in Derbyshire it affords sulphur and copper in the form of pyrites.

The stone usually called alabaster, employed in making statues and ornaments of sculpture, is properly a carbonated lime, nearly allied to marble, though it is usually considered to be a variety of gypsum. Limestone alabaster is commonly found in blocks in marble quarries, as in the Greek island Paros, in Tuscany, and some other districts of Italy.

2. Gray Wacke consists of fragments of quartz and argillaceous schistus, cemented together by a clayey substance. This stone is distinctly stratified, but the strata are not found to be parallel to those of the other rocks on which they rest; it is commonly observed covering limestone, especially at the foot of hills.

Gray wacke often furnishes slate of a good quality, and it is rich in metals: the greater part of the veins of lead and silver in the Hartz forest, in Germany, lie in gray wacke; in Transylvania it contains rich mines of gold.

3. Secondary trap. In secondary strata are found several ..varieties of trap, all composed of hornblende and feldspar

very intimately mixed, the grains of each being so fine that the substance seems to consist of only one material.

4. Sandstone, or grit. Under these terms are comprehended various sorts of stone, the calcareous, the argillaceous, the siliceous sandstone. In these the component materials are quartz, with a little schistus and feldspar, in the form of small grains cemented together by means of flint, clay, or limestone. The argillaceous standstone is found in immense beds very distinctly stratified and divided by fissures into the shape of a parallelopiped. This is one of the most abundant productions of nature, occurring in almost every country: it usually contains petrifactions in abundance, but seldom affords any metals; cobalt has however been found in it. The siliceous sandstone contains many petrifactions, but no metals have hitherto been discovered in it.

5. Gypsum, or plaster stone, is a native combination of lime with sulphuric acid, commonly called oil of vitriol, because it was obtained by distillation from that substance. The common gypsum is a compact granulated stone, usually of a grayish colour, containing a quantity of carbonate of lime: its texture resembles coarse loaf sugar. This substance is abundant in many parts: the hill of Montmartre rising over the north side of Paris, consists of this gypsum, which is thence, even in this country, usually called plaster of Paris. It contains petrifactions, and abounds with the impressions of animal and vegetable substances: copper has been found in it, but in general it produces few metals. Though, from the ordinary form or situation of gypsum and the remains of organised bodies it contains, no doubt can be entertained of its being in most cases a secondary substance; yet from its having in some instances been found mixed with mica, some naturalists have ranked it among the primitive compounds.

6. Fluor spar. This beautiful substance is a native compound

pound of lime with a peculiar acid called fluoric acid, is found in large blocks, and also crystallised in cubes or octahedrons; it is of different colours, but the most common varieties are those in parallel bands of green, blue, yellow, and white, or with a white ground veined with a reddish brown. Specimens have occasionally been met with shaded so as to resemble a map. This substance is so soft as to be easily cut and turned in a lathe into vases and other ornaments commonly seen on chimney pieces and side tables. Fluor spar is found in many parts of the world: it is not uncommon in France, and in Britain the mines near Castleton, in Derbyshire, are the most productive, whence it is often called Derbyshire marble. The fluor commonly rests upon limestone; and it frequently has limestone for a nucleus or kernel, around which it appears to have crystallized in some parts of the mine the fluor is found in detached lumps, appearing as if they had been attached to and broken off from some other body. Pieces have now and then been found a foot in thickness, with four or five bands; but such pieces are very rare, the general thickness being three or four inches.

Fluor spar is found in the north of Scotland in mountains of granite, and in the Shetland Isles in a vein of basalt.

7. Chalk is usually found in horizontal beds, often many yards in thickness, reposing on layers of other calcareous stone of a harder structure. These beds are often of a great extent, very commonly containing layers of flints and vast quantities of shells.

Chalk, which is so abundant in some countries, is rarely found in some others, as in mountainous countries; thus in Scotland it is very rare, whereas in the southern and south-eastern parts of England it is very common: the south and west of France also afford vast cliffs and beds of chalk. If a line be drawn from Dorchester in Dorsetshire to the county of Norfolk, it will form the boundary of the great

great chalky stratum of England; very little having ever been found to the westward or northward of such a line.

No metals have ever been met with in chalk, although it be said that martial pyrites, a compound of sulphur and iron, has been discovered in it in France.

8. Clay is found in various states with respect to hardness or solidity, from the soft ductile clay employed by potters and pipemakers to the perfect argillaceous schistus or clay slate, already described.,

Soft clay is found in beds of various thickness, in common not far below the surface, in alternate strata with harder clay, slate, sand, or limestone: it is in general very abundant, especially in tracts where coal or rock salt is found.

Clay of a harder consistency, called indurated clay, is usually found below the soft, and sometimes a stratum of slate is interposed: petrifactions and shells are often found in it.

A harder state of clay forms lithomarga, or stone clay, which is found in beds alternating with the former or with limestone, especially in coal mines: it sometimes presents the impressions of reeds and other vegetable substances.

The next degree of hardness in clay forms slate clay, and one still harder gives slate or schistus: this is of a dark brown or blackish colour, and of a texture capable of being divided into many laminæ, or thin parallel plates: it is common in coal countries below the sandstone, or alternating with it and limestone: it often bears impressions of organic remains, and sometimes contains lead ore.

Nearly allied to this substance is what the miners call rubble stone, which is a common variety of slate found in similar situations with slate itself, but often very rich in metals, especially iron, bismuth, and cobalt; it also abounds in petrifactions, and is sometimes found in primitive rocks.

9. Marl

9. Marl consists chiefly of sand, clay, and calcareous matter: it is found in many places, and forms one of the most valuable natural manures used in agriculture: it is of various degrees of solidity, from a soft powder to the consistence of stone: in colour it is generally of a reddish white, but not unfrequently of a yellowish brown or blackish cast. Marl is usually deposited in considerable beds of various. degrees of thickness in vallies and other low lands, especially among coal strata.

10. Argillaceous ironstone. This is very common in coal countries it is very compact and heavy, and of various colours from a dark brown to a blood red, the latter forming the hematites or bloodstone, one of the richest iron ores: it often contains little spherical bells like bullets. This ironstone is disposed in strata alternating with indurated clay, slate clay, marl, or sandstone, seldom far below the surface; in general it forms beds of small extent, and is often confined to particular spots. Ironstone is found in great abundance in Cumberland, and in the most parts of Scotland; it may be seen laid open all along the cliffs on greatest part of the coast of Fife.

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11. Wucke and basalt. These substances are very nearly allied to trap, and have been classed with it under the general name of Whinstone. Wacke differs from trap only in being more compact and of a finer grain; it is heavy and so hard as often to strike fire with steel: the colour is generally a reddish brown, or a gray of various shades: it often forms a considerable part of mountains, either in vast blocks, as in the hill on which Edinburgh castle is placed, or in strata alternating with limestone or sandstone.

Basalt has a finer grain and is more compact than even wacke, being the densest of all the traps or whinstone; it is found in large blocks covering other strata in the form of tables, or in regular primitive columns, straight or bent. Basalt is the substance composing the beautiful columns supporting