sition and research, that in the course of no long period a professor of each will appear in universities; and each might occupy the sole pursuit of an author who is zealous to make discoveries, or to compose complete and classical works. One of the chief causes of the slow progress of the science is, that it is too wide for one mind; and as zoology has been divided into ornithology, ichthyology, entomology, &c. so mineralogy, to be duly studied, should have grand subdivisions.

HIS DOMAINS.

These provinces may again be viewed as divided into DOMAINS, corresponding with the Orders of some writers and the Genera of others, as the Provinces supply what are called Classes. This term DOMAIN is preferred to District, &c. as it not only implies a subdivision of a province, but, in another acceptation, a ruling or preponderating power, strictly applicable in mineralogy, where it is often the preponderance, and not the universality, which imparts the denomination. Thus in the siliceous, calcareous, and other domains, it is only understood that the denominating portion preponderates, as few or no rocks are pure, and bnmixed with other substances.

Petralogy, a province of mineralogy, may therefore be regarded as divided into Twelve Domains; of which the first six, being distinguished by the substances themselves, may be called SUBSTANTIAL: while the remaining six, being distinguished by circumstances or accidences of various kinds, may be called ciRCUMSTANTIAL, OF ACCIDENTAL; but this last division is of little moment.

The first six domains of Petralogy comprise, 1. The Siderous Rocks, or those in which iron predominates, not in the comparative quantity when analysed, but in the quality and essential difference which it imparts. 2. The Siliceous, denominated as usual from the quantity of Silex. 3. The Argillaceous. 4. The Magnesian: these two are again den minated from predominance. 5. The Calcarcous. 6.

The Carbonaceous.

The remaining six domains, derived from circunstances or accidences, are, 7. The Composite, or Aggregated Rocks, as calcareous spar with schorl, quartz, and garnets, felspar and siderite or hornblende, &c. This domain has often been confounded with the granites, however alien from that description. 8. The Diamictonic, or rocks in which the substances are so completely mingled, that it is difficult, even upon an analysis, to

pronounce which preponderates. 9. Thễ Anomalous, or those which contradict the conumon order of nature, and present unexpected and unusual combinations. Some of these domains, though they afford few objects at present, may, in the progress of the science, be greatly enriched and enlarged; and the utility of such divisions will be more perceptible as the study advances towards perfection, the greatest obscurity at present arising from the want of necessary subdivi sions.

The remaining three domains are ge. nerally admitted in geological works, namely, 10. The Transilient Rocks, an interesting series, in which one substance gradually passes into another, as granite into porphyry, trap into wacken, and the like. 11. The Decomposed Rocks, which gradually decay into sand, clay, or productive soil. 12. The Volcanic, which require no other description.

HIS MODES.

Having thus established the Domains, or Great Divisions, of Petralogy, the smaller distinctions, can be derived only from the objects, themselves, as we now arrive at what are by most mineralogic authors denominated Species, though in their arbitrary and unnatural systems, as Dr. Townson has observed, the Genera and Species are often confounded. “Thus in the improved edition of Linnæus, the characters which constitute the Species in gypsum form Genera in the carbonate of lime; for the pulverulent, fibrous, spa. thous, and compact kinds of gypsum form but so many Species, whilst the pulverolent, fibrous, spathous, and compact kinds of carbonate of lime form so many different Genera." Now these very appearances, which constitute the arbitrary Species and Genera of former authors, what would they be, in the eyes of a phiJosopher or grainmarian, except different modifications or modalities, of the same substance, and which by a shorter term may be denominated Modes? Hence the term McDE, which is universally ap plicable and unobjectionable, to distin. guish such objects, in mineralogy, is here admitted instead of Species.

To put the propriety of this new appellation to the test, examples may be produced of what are called Species by the most celebrated mineralogic writers. Wallerius, among the species of garnet, first mentions that of an undetermined figure, composed of granular particles; and his next species is of an undeter mined figure, but laminar. - What are

these

the difference of chemical composition, &c. and the Structure the grand characteristic, the Aspect refers to more minute features.

HIS VARIETIES.

The term variety is unobjectionable, as it is equally applicable to objects of animated or inert matter; and diversity may be used to imply a still greater difference than the variety presents. A very faint shade of difference might, if necessary, be called a lineament.

THE WERNERIAN THEORY.

From the sketch imparted by Daubuis son to Brochant, and from Mr. Jameson's Geognosy, we are enabled to form an idea of Mr. Werner's system concern. ing the formation of such parts of this planet, as we can hope to observe, little exceeding the three thousandth part of its semi-diameter, I warmly subscribe to the sentiments of admiration which are paid to Mr. Werner's superior talents in many branches of mineralogy, a science infinitely indebted to his industry and sagacity. I also acknowledge the truth of the apophthegm, Natura fecit omnes judices; paucos artifices. But I regret, with his most enlightened admirers, that the scene of his enquiry has been too confined; and that his view of the mountains of Saxony has not been extended over the globe. After forty years of sedulous observation among the Alps, Saussure, who began his labours with a view of forming a system, declares that his hopes were frustrated; and that he had met with such unaccountable confusion that he could not venture to propose a theory, Yet Saussure, to practical observations on a far superior scene, added the advantages of learning, and mathematical and meteorological science, which Mr. Wer er unhappily wants, and which would have corrected and greatly improved his speculations.

Domain I.-Siderous,
SIDEGEA, SIDEROUS EARTH.

The name sidegea, as not unusual in compounded words, is abbreviated from two Greek terms, signifying iron and earth. The reasons for the introduction of this grand division, adopted in substance by the most eminent geologists, have already been given. Iron acts so important and radical a part in the constitution of our planet, that it deserves to be viewed under various aspects, not only as a metal, but as an earth, strongly

impregnating most of the others, and often exerting a predominating influence. For as, since the recent discoveries, many earths have been known to assume the form of metals, so there can be no impropriety in considering this universal metal under the form of an earth.

When a substance contains more than twenty-five parts in the hundred, or, in other words, one quarter, of iron, it may be worked as a metallic ore, and arranged under that denomination. But in a smaller quantity it will fall under the present division, especially when intimately combined with the other earths. It was by metallogists considered as a calx, or latterly called an oxyd. Mr. Kirwan, who has rightly added calces of iron to his description of the earths, says, that they are formed of that metal, com bined with different proportions of pure air, and frequently of water also and fixed air.

"One hundred parts metallic iron are capable of taking up 66 or 70 of pure air. When 100 parts iron contain but 40 of this air, the compound is still magnetic. His table of the fusibility of the simple earths presents some curious experiments on the mixture of calcined iron and rust of iron, with other substance, which show the power of this metal. Even when it only amounts to four parts in the hundred, it sensibly influences the compound.

Sidegea, or siderous earth, is so gene rally diffused, that almost every mineral substance derives its colour from it, from a pale blue to the deepest red. Animal substances contain it; and it exists in the vegetable kingdom, even in plants apparently supported merely by air and water. It would appear that even the atmosphere abounds with atoms of iron, whence perhaps the meteoric stones.

Domain II.-Siliceous.

SILEX, SILICA, OR SILICEOUS EARTH.

This earth derives its name from the silex, or flint, in which it abounds. Some also denominate it quartzose earth, be cause it is perhaps more abundant in the stone called quartz, which, when transparent and crystallized, is styled rock crystal. It so frequently occurs in the form of sand, which covers a great part, of the globe, either alone or mixed with clay, that late chemists infer that such sand arises not only from the decompo sition of rocks, but is often a disturbed or hasty crystallization of silica. This is further confirmed by the circumstance

that many primitive mountains consist of granular quartz, of an arenaceous appearance, like agglutinated sand.

The stones now called siliceous, were formerly denominated vetrifiable; because, with an alkali, they may be melted into glass; and the finest Venetian glass was fabricated from quartz, by the Italians called tarso. Silica, like the other simple earths, is a fine white powder; but the particles have a harsh feel, like minute sand. Alone it is scarcely fusible; but, when newly precipitated, is soJuble in 1000 parts of water.

Joined with iron, argil, and magnesia, it constitutes the primitive and most important rocks, rising to the regions of perpetual snow, and thus supplying unfailing aliment to the great rivers that fertilise the earth. When considered in these mountains, in sand, and in clay, it may be pronounced the most abun dant of all the earths; and, if iron form the nucleus, the shell of this planet may be said to consist chiefly of silex. It is suspected that it is coeval and intimately connected with iron; as the aerolites or meteoric stones, and the large masses of native iron, discovered in Siberia and South America, contain abundance of silex mixed with some magnesia.

Siliceous substances generally strike fire with steel; and flint or quartz yields a peculiar odour, supposed by some to arise from a subtile substance which chemistry has not been able to discover. A strong phosphorescence is also produced by collision, so that, during Alpine hurricanes, the torrents, rolling large fragments of rock, present a singular scene of corruscation.

Domain III.-Argillaceous.

ARGIL.

This earth is obtained in the state of greatest purity from alum, which is a mixture of argil and sulphuric acid. If it contained oxyd of iron, as is frequently the case, it emits a particular smell, when breathed upon, well known by the name of an earthy smell.

With heat it loses its water, and diminishes in bulk; but a very violent heat converts it into a white amel. When combined with line, it easily enters into fusion.

Argil, also called Alumina by recent chemists, is of great utility, as forming the basis of many manufactures, such as brick, porcelain, and earthenware. It constitutes 98 parts in the 100 of corindon; under which division are now classed the most perfect of the precious stones, af

ter the diamond, such as the sapphire, ruby, and oriental topaz. It is hence not only one of the most noble, but one of the most useful of the earths; loam or fertile soil being a mixture of about 30 parts argil with 70 of fine sand; while mould chiefly consists of animal and vegetable remains.

In the primitive rocks argil is an important feature, forming about a fifth part of felspar, and a third of mica. The most ancient slates abound in argil. It is often so homogeneous that it cannot be regarded as the waste of former mountains, but a pore deposit of primeval waters, In the primitive schisti however there is still a great preponderance of sand; and the glossy appearance may sometimes proceed from decomposed mica,

The argillaceous rocks are mostly of simple and uniform appearance, and do not admit the numerous modifications of some other substances. This earth is chiefly eminent in gemmology, where it constitutes some of the most beautiful va rieties. The argillaceous rocks are never crystallised, and present but small splendour in their appearance; hence they are very seldom used in the ornamental arts, and are chiefly important in a geological point of view, where they often rank among the most important primitive substances. Yet even in this light they have not been treated with the attention and minute investigation which have been bestowed upon the Siliceous and Calcareous Divisions,

The essential part of the argillaceous rocks being alum, it seems the most natural progress to begin with those substances which chiefly supply commerce

with that earth.

Domain IV.-Talcous.

MAGNESIA.

This earth seems first to have been discovered, or at least sold as a remedy, by an ecclesiastic at Rome about the beginning of the eighteenth century. Under the name of magnesia albu, it was proposed as a universal medicine, while it could do little more than supply the place of the Lemnian earth, and other boles. As Theophrastus, however, in describing the stone called magnetes, says it may be turned on a lathe, and has a silvery appearance, Dr. Kidd agrees with Hill, that the ancient Greeks called the load-stone heraclea, but the more modern magnetes: and Pliny's descrip tion of the stone brought from Magnesia,

in Asia, seems to belong to a talcous substance.

Hoffinan, Black, and Bergman, contributed to establish the difference be tween magnesia and lime. It seems ori ginally to have been prepared from ni. tre; but sea-water contains the sulphate of magnesia, a salt composed of this earth and sulphuric acid; and which is also found in many springs, particularly at Epsom, whence it was called Epsom

salt.

Magnesian or talcous earth is infusi ble in the strongest heat. It does not form phosphorets, like the three other alkaline earths, lime, barytes, and strontia.

In tale it sometimes amounts to one half of the composition; but in the other substances, such as steatites and serpentines, it is only from twenty to forty; but its power is so great as sensibly to alter the appearance and qualities of the stone. The chrysolite or peridot of the French, containing about one half maguesia, belongs to this division; and is remarkable as the only magnesian gem. The deserts of Siberia are annually covered with efflorescences of Epsom salt, so as in the short summer to resemble \ snow. The talcous rocks in general present a discriminating character in their unctuous appearance; they have however, in some cases, been confounded with the argillaceous, which occasionally assume the softness and silky lustre of the magnesian. The presence of magnesia is often indicated by a green coJour.

Domain Y.-Calcareous.
CALCAREOUS EARTH.

This important substance is produced by burning limestone, marble, or chalk; and is commonly known by the name of lime. The purest is yielded by calcareous spar, or some white marbles.

Its taste is hot and acrid; and it is incapable of fusion, even by the burning glass. It may however be fused when joined with silex or clay.

Limestone is composed of lime and carbonic acid. Ileat separates the latter, and the lime is left pure. This acid is a species of gas, formerly called fixed air, and discovered by Dr. Black in 1756; an event which formed a revolution in the history of chemistry. Atmospheric air is composed of about seventy-four parts in the hundred of nitrogen, and twentysix of oxygen: but the latter varies; and there is commonly one in the hundred of MONTHLY MAG. No. 229.

P

When combined with sulphuric acid, the calcareous earth forms gypsum, or selenite, which being burnt produces what is called plaister of Paris. The alabaster of the moderns commonly belongs to the same combination; while that of the ancients is often a stalagmite, or secre tion of common limestone. With fluoric acid, calcareous earth becomes fluor or fluate of lime.

The greater proportion of limestone is produced by the decomposition of marine shells; but the more ancient, which is crystallised, and presents no trace of such remains, is called primitive, being supposed as ancient as any of the rocks. It is in general easily distinguished from the other substances by the nitrous acid, formerly called aqua-fortis, which excites effervescence; but when mixed with magnesia, or much silex, this effect is slowly procured. Nor do gypsum nor fluor etfervesce.

To these observations, which are chiefly extracted from Kirwan, Thomson, and Patrin, it may be added that, in 1808, Mr. Davy reduced lime to a metal, which had the colour and lustre of silver, and burnt with an intense white light into quick lime.

In some works of mineralogy the first three Modes of this Domain, and even the three succeeding, have been arranged as mere sub-species, or varieties of linestone. Strict chemical analysis may probably discover a different proportion of ingredients, as for examples, more water of crystallisation in marble, and more or less silex or argil; and there is at any rate a difference in the mode of combination. But the chief use of any system being to assist the memory, even the strict precision of terms becomes mere pedantry, if it be not subservient to this main object. Too large masses of colour,

or

too small, will render the picture equally inelegant and obscure.