OBSERVATIONS.

The author has attempted, in this chapter, to give a succinct account of the geological distribution of fossil organic remains, belonging to the animal and vegetable kingdoms. This, he conceives, will interest the learner, for whose use it was chiefly intended, more than a detailed enumeration of the genera or species supposed to be peculiar to different rock formations. With respect to fossil conchology, he is inclined to believe, that the attempt to identify the strata of distant countries by the isolated occurrence of any particular species of shell, has been carried farther than a sound induction from facts or analogy would warrant. His opinion on this subject, given in the second edition of this work, he will here insert:-"It may be doubted, whether the occurrence of similar organic remains is sufficient to identify strata in distant parts of the globe; for, could we admit that strata are universal formations, and extended from the frozen to the torrid zone, it seems more than probable, that the animals which lived on any one particular stratum, would be of very different species in different latitudes."-We know so little respecting the forms or habits of the animals classed by the conchologist, that we are far from being certain whether many shells which he regards as belonging to different species, or even genera, are not mere varieties of form, occasioned by difference of age or situation. Such a change is ascertained to take place by age, in shells of the genus Cypræa.

In animals like the mollusca, which have no internal skeleton to determine their form, the construction of the external shell may, probably, admit of considerable variation under a change of circumstances. Few conchologists, excepting M. D'Avilla, have made accurate observations on the living animals inhabiting oceanic shells. His interesting work, entitled "L'Histoire Naturelle éclaircie dans une de ses parties principales, la Conchologie; et augmentée de la Zoomorphose, ou Representation des Animaux à coquilles, avec leurs Explications," presents us with some truly extraordinary forms of molJuscous animals, of which we could not have had a remote notion from the mere study of the shell.

In strata belonging to one formation, and in adjacent districts, the existence of certain shells, whether we regard them as distinct species or as varieties, may be of use, in identifying any particular bed-and in distant countries, where we find the same remarkable species of shell associated with any other remarkable species in considerable numbers, it may serve to identify a particular rock formation, where the mineral character of the rock may be very different from that in which the observer has been accustomed to meet with them. The occurrence of a considerable number of Gryphææ, the Gryphæa arcuata, in a bed of blue clay in the mountains round the Lake of Annecy, in Savoy, served the author as a key to discover to what formation the calcareous strata belonged, when their mineral characters would have indicated a more ancient series.

Vegetable organic remains have not, till recently, been studied with the attention which they deserve and require.-These remains are never found entire, as is frequently the case with the skeletons and

exuviæ of animals: they cannot, therefore, be satisfactorily studied in cabinet collections. Were any botanist, well acquainted with vegetable physiology, to devote some time to exploring the vegetable remains, as they are abundantly brought up by our coal-miners, they might have the opportunity of re-constructing many entire species from the fragments; but, in order to form a geological classification of fossil plants, a practical acquaintance with all the secondary strata is further required. The attempt of M. Adolphe Brongniart to give a geological classification of the families of plants peculiar to each principal formation, is entitled to high commendation, as the nearest approximation to a correct arrangement of fossil plants that has yet been made. See his "Histoire des Végétaux fossiles," and his "Prodrome d'un Histoire des Végétaux fossiles."

CHAPTER 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,-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.

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, 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 into 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 argillaceous; 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 differ from each other only by different degrees of hardness or of crystallization. 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

* 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 Es

says.

specific gravity in all stones or earthy minerals if it much exceed 2.5, or approach 3 may be attributed in other words, if they are nearly three times heavier than an equal bulk of water. Gems 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 well-known 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 even in 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 vegetables 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 combi