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CHAP. III.

ON THE MINERAL SUBSTANCES THAT COMPOSE THE CRUST OF THE GLOBE; AND ON THE STRUCTURE OF ROCKS.

The constituent Elements of the simple Minerals that compose Rocks. The physical Characters of simple Minerals composing Rocks. Explanation of the Terms employed in describing the internal Structure of Rocks, and the external Structure of Mountain Masses. Sedimentary Depositions.

THE most careless observer can scarcely fail to notice, that the mineral substances which occur on the surface of the globe differ from each other in density, hardness, colour, and other sensible qualities. Indeed, the different varieties of stone appear at first so numerous, as to render it difficult to become acquainted with them: but, however numerous these varieties may be thought, the simple minerals which compose rocks or strata are very few, and the elementary substances of which each of these minerals is formed are still fewer. *

The elementary substances of which the solid matter of our globe is composed, are the Earths, ---

* The mineralogist and the geologist consider those minerals as simple and homogeneous, which present no difference of qualities to our senses throughout the mass, although the chemist may discover that such minerals are composed of two or more elementary substances. Thus, limestone or marble is regarded as a simple substance, though chemistry has discovered that it contains, in every 100 parts, lime 57 parts, and carbonic acid 43. It is the latter which is expelled from it by burning; a process which is well known to make the stone lighter, and to render it caustic; in which state it is called quicklime. Nor do the researches of the chemist end here: the two substances, quicklime or pure lime, and carbonic acid, are themselves compounds: the former, lime, is a compound of a metallic substance called calcium, united with oxygen; the latter, or carbonic acid, is composed of oxygen and carbon or charcoal.

silex, alumine, lime and magnesia. The Metals, iron and manganese. The Inflammable Principles, carbon and sulphur; and the Alkalies, - potash and soda. Muriatic and Phosphoric Acid occur also in the mineral kingdom. The newly discovered earths and alkalies, and metallic ores cannot be regarded as forming essential constituent parts of rocks: they chiefly occur in veins. The four earths above enumerated, together with iron, compose nineteen parts in twenty of the known solid matter of the globe. The Earths, when pure, are infusible, except at an intense heat; they are nearly insoluble in water at the common temperature: when pure, they are white or colourless. Though the earths are infusible when pure, if they are combined in certain proportions, they may be fused with facility at a comparatively low temperature.

Silex, or Siliceous Earth, exists nearly pure in large masses, forming minerals, and even entire rocks, as rock crystal, quartz rock, and flint: it communicates a great degree of hardness to all rocks or stones in which it enters in a large proportion. Such stones are denominated Siliceous: they resist the point of a knife, or scratch glass. In its combinations with other earths, Silex appears to act as an acid. More than one half of the crust of the globe is composed of siliceous earth either pure or combined. In some thermal waters, siliceous earth occurs either in a state of minute division or in solution; and the waters of the boiling springs, or geysers, in Iceland, deposit siliceous incrustations of considerable thickness.

Alumine, pure argillaceous Earth, (Lat. argilla, Fr. argille,) is a substance which in a mixed state is well known, but pure unmixed clay is one of the rarest substances in the mineral kingdom. This earth is soft, smooth, and unctuous to the touch; it strongly absorbs water; where it exists in the proportion of thirty per cent, it communicates in some degree these properties: such rocks are called argil

laceous; they generally contain a notable portion of iron, which appears to have a greater affinity for this earth than for any other.*

Lime (Lat. calx, Fr. chaux) is a well-known earth combined with carbonic acid, in which state it forms limestone, marble, and chalk: these only differ from each other by different degrees of hardness or of crystallisation. Mountains composed of lime are denominated calcareous. When lime is united with sulphuric acid, it forms the stone called gypsum, which is softer than limestone, and does not, like it, effervesce with acids. Calcareous earth mixed with common clay forms marl.

Magnesia has rarely been found pure in a native state. It enters into the composition of some of the primary rocks, to which it generally communicates a soapy feel, a striated or striped texture, and sometimes a greenish colour. It occurs also in various limestones in different proportions.

Iron appears to be more abundant than magnesian earth it forms a constituent part of numerous rocks and stones; to it they most frequently owe their colour: the earths, when pure, are white. Iron, when in combination with the earths, is, like them, an oxide, or a metal united with oxygen. To the presence of iron the increase of specific gravity in all stones or earthy minerals may be attributed, if it much exceed 2.5, or approach 3: in other words, if they are

* Though alumine or pure clay communicates a soft quality to most stones of which it forms a principal constituent part, a very remarkable exception to this is offered in adamantine spar and the sapphire, which nearly equal the diamond in hardness. Klaproth, one of the most laborious and eminent chemists of the present age, has analysed these stones: the former contains 90 parts in the 100 of pure clay; the latter 95 parts in the same quantity. “What a high degree of cohesive power (he observes) must nature command, to be able to transform such a common substance as clay (aluminous earth) into a body so eminently distinguished and ennobled as the sapphire by its hardness, brilliancy, and its resistance to the action of fire, of acids, or the effects of all-destroying time!" -Klaproth's Essays.

nearly three times heavier than an equal bulk of waGems and the earths barytes and strontian are exceptions; but these never form entire rocks. The presence of iron not only increases the weight, and darkens the colour of numerous rocks and stones, but is one principal means of their decomposition, for iron exists in stones in two states of oxygenation, as the black or the red oxide; and when the former is exposed to air and moisture, it absorbs a greater portion of oxygen, and is converted into a brown ochrey incrustation, which peels off, and exposes a fresh surface of the stone to a similar process.

Manganese, in a state of oxide, occurs in a few rocks, to which it generally communicates a dull reddish colour inclining to purple, and a peculiarly dry and burnt-like appearance.

Sulphur, though found in considerable masses, cannot by itself be regarded as a constituent part of rocks; but when it is combined with oxygen forming sulphuric acid, it unites with lime, and forms the wellknown mineral gypsum or plaster stone.

Carbon, or Charcoal, enters as a constituent part into many of the slate rocks, to which it generally communicates a dark colour: it forms also regular beds of considerable thickness, being the principal constituent part of coal. Carbon, combined with oxygen, forms carbonic acid or fixed air, which is combined and solidified in all limestone rocks in a proportion exceeding two fifths of the whole weight. As carbon exists in such a large proportion in even the oldest limestones, we may regard it as a constituent element, and not as a substance derived from the vegetable kingdom. For whence did the vege tables themselves derive their carbon?

Potass and Soda. — These alkalies occur in minerals which compose parts both of primary and volcanic rocks; but the proportion is so small, that they would scarcely deserve the attention of the geologist, did not the latter alkali, soda, exist in such abundance in the waters of the ocean and in rock salt. Pure sea

salt, or rock salt, contains nearly 53 parts of soda, 46 muriatic acid or chlorine.

Muriatic acid, combined with soda, is the only state in which this acid forms a constituent part of any rocks we are yet acquainted with; except in some volcanic rocks, where it may be regarded as accidental.

Phosphoric Acid, combined with calcareous earth, is a principal constituent of animal bones: it occurs also in a few limestone beds, which are supposed to have derived phosphoric acid from the decomposition of animal matter. This acid is of very rare occurrence in the mineral kingdom.

The above elementary substances, either separately or combined, form all the simple minerals of which rocks are composed. A knowledge of these minerals, and their different intermixtures and combinations, can only be learned by an examintion of specimens: they are, however, far from being numerous; and a short description of each is necessary in an introductory treatise.

The most important simple minerals composing rocks are quartz, felspar, mica, talc, chlorite, hornblende, serpentine, limestone, and slate.

Quartz is one of the hardest minerals of which mountain masses are composed: it gives plentiful sparks with steel; it breaks with a smart stroke of the hammer; the surface of the fracture in crystallised quartz is conchoidal, in uncrystallised splintery: the lustre is vitreous. Crystals of quartz, or rock crystals, as they are commonly denominated, have different degrees of transparency: the blue varieties are amethysts. The most common forms of the crystals are six-sided prisms terminated by six-sided pyramids; or, two six-sided pyramids united, forming a dodecahedron, whose faces are isosceles triangles. Uncrystallised quartz is seldom transparent, most frequently translucent, but sometimes opaque. Its colours are various shades of white, grey, brown, yellow, red, and green. It yields a phosphorescent