IX. Hyaline quartz, without ever forming the entire sub- BOOK, stance of any mountain, abounds in every soil. It is one of the integral parts of most varieties of the granite rocks. Situation It also enters, in the form of crystallized grains, into the of hyaline composition of many rocks of porphyry. It forms the base quartz. of a great number of fossil micaceous rocks. Its crystals Occupy the fortuitous cavities which are found in almost all the large masses of rocks. They ornament the walls or sides of such cavities, sometimes in transparent prisms of nearly a foot in diameter. Hyaline quartz forms veins, frequently of a great extent, which pass through the primitive mountains; and these veins, becoming exposed, and projecting by the wearing away of the rocks in which they were embedded, have given rise to the opinion, that there existed mountains entirely composed of quartz. There is scarcely any secondary rock, in which we do not find com- / mon or unshapen quartz, in masses, in veins, or in crystals.* of arena ted quartz. Arenaceous quartz, (quartz arénacé) or quartz in the Situation form of sand, covers almost entirely the bottom of the sea. It is spread over the banks of rivers, and forms vast plains, even at a very considerable elevation above the level of the sea, as the desert of Sahara in Africa, of Cobi in Asia, and many others. This quartz is produced, at least in part, from the disintegration of the primitive granite rocks. The currents of water carry it along, and when it is in very small, light, and rounded grains, even the wind transports it from one place to another. The hills thus are made to move like the waves, and a deluge of sand frequently inundates the neighbouring country.f Arenaceous quartz furnishes, by fusion, one of the most useful substances we have, namely, glass, which, being less hard than the crystals of quartz, can be made equally transparent, and is equally serviceable to our wants and to our pleasures. There it shines in walls of crystal in the palaces *Brogniart, ii. 271–279. So where o'er wide Numidian wastes extend, Sudden the impetuous hurricanes descend, &c.-See Addison's Cato. 1x. BOOK of the great, reflecting the charms of a hundred assembled beauties; here, in the hand of the philosopher, it discovers to us the worlds that revolve above us in the immensity of space, and the no less astonishing wonders that we tread beneath our feet. Precious stones. We shall now shortly notice the different species of precious stones, which are almost all composed of alumine, or pure argil, as the analysis of Bergmann and Klaproth have proved. According to the method of Haüy, the name of Corundum now comprehends a species, the red variety of which is the true oriental ruby ;* the blue, the oriental sapphire; and, the yellow, the oriental topaz. These three varieties are comprehended by the French amateurs under the general name of "oriental gems." These valuable and brilliant substances consist of ninety-eight parts of alumina, and two of iron. To the species called by the French Spinelle. Spinelle, the colouring matter of which is the metal called chrome, belong the scarlet or light red ruby, the pale rose or pink ruby, the variety, called rubacelle, of a yellowish red colour: all these are less hard, and have less play or brilliancy than the corundums. The limpid and transparent topazes of Siberia, the pale yellow ones of Saxony, those of Brazil, the colour of which is a reddish yellow, are all included by Hay in the same class. The ruby of Brazil is only a red topaz, (sometimes reddened by means of fire,) the beryl or oriental aigue marine, called the sapphire of Brazil, is a greenish blue topaz. Many chrysolites beEmerald. long to the same species. The beautiful emerald of Peru, the pure green of which is more pleasant to the eye than the dazzling brilliancy of many other more perfect gems; and the occidental or common beryl, a stone of but little value, have both the same bases, namely, silex, alumina, glucine, and lime; but the colouring principle in the emerald is chrome, and, in the beryl, a very small quantity of iron. The garnets of Bohemia, of a deep bright red, and Topaz. Garnet. Lapidaries call all perfect gems oriental, and those which are less so occi dental. IX. those of Syria, of a purple violet colour, contain a large BOOK proportion of iron, sometimes one-third, or even two-fifths: the oriental garnet is very magnetic. It is a singular circumstance, that this quantity of iron does not injure its transparency.* The cymophane, known also by the names of chrysoberyl, and oriental chrysolite, is of a yellowish green colour, and is nearly as hard and heavy as the corundum. We have not yet spoken of the diamond, that king of Diamond. gems. The reason is, that this king is now dethroned. Modern chemistry has proved, by multiplied and decisive experiments, that the diamond, far from resisting the fire, like all true gems, is entirely dissipated, without leaving any residuum whatsoever. Consequently, the diamond is now classed among the combustible substances, along with sulphur, amber, and coal. It appears that the diamond consists of pure carbon. Like all the other fine gems, the diamond seems to abound most in the East Indies, and in South America. The precious metals also more peculiarly belong to the equatorial regions. Passing over some less interesting species, we come to Feldspar. feldspar, a substance composed principally of silex and alumina, with small proportions of lime and potash. If it be coloured, it is by the presence of oxide of iron. It cuts glass, is phosphoric, and emits sparks when struck with steel. The feldspar forms the base of a multitude of rocks, and predominates in those of primitive formation, constituting at least two-thirds of the substance of granite. Extensive mountains are sometimes solely composed of it. Guldenstedt tells us, that feldspar, either pure, or mixed with granulated quartz, forms that vast plain of rocks, which extends from both sides of the cataracts of the Dnieper. The fosses of the fort of Sacharowa are cut out of natural feldspar. It is also to this substance that porphy *Haüy, ii. 551. + Dolomieu, quoted by Haüy, ii, 608. Georgi, Russie, iii. 179. BOOK IX. ry rocks owe the distinct spots which arise out of their general colour; but these rocks rarely present themselves under regular forms. The fine crystals of feldspar. whether opaque and coloured, or limpid and transparent, occupy veins or cavities, contained in the primitive mountains; and it is the Lombard Alps which have furnished what the cabinets of France consider the most perfect specimens of this kind. But the most beautiful crystals of feldspar, which join to a fine green colour a great degree of transparency, are found in detached blocks or masses in the steppes of the Kirguis, whence the Bucharians carry them to Semipalatnoi.* It is the mountains of Siberia, towards the lake Baïkal, which have supplied these large plates of azure feldspar, with which the palace of Czarskoselo is adorned. It appears then that this substance abounds still more in the Alps of Asia than in those of Europe. On the contrary, America does not appear to afford it in large quantities. Decompos- Feldspar, even when decomposed, still maintains a character of importance. It is found in extensive beds, from the Uralian mountains to Kamtchatka. Of the two substances which the Chinese use in the making of porcelain, the one named petunze, is a whitish laminated feldspar; the other called kaolin, is an argiliform feldspar, that is to say, feldspar which has passed by decomposition, from the state of a stone to that of a very bristle clay, without cohesion, combining with water, of a fine white colour, and infusible by itself the petunz acting as a tlux. The same substances are employed in Europe in the manufacture of porcelain ed feldspar, ware. Petrosiles. The name of petrosilex has been given to several substances, very widely distributed in nature; but there appears to exist great confusion in this subdivision of the mineral kingdom. The hornstone of Werner or keratolithe Pallas, Nouv. Mémoires du Nord, v. 309. Bindheim, Analyse, etc. dans les Mémoires des Naturalistes de Berlin, vol. xi. Petrosilex, a Latin word, which corresponds to the words, fels-kisel in German; berg-flint in Danish; helle-flinta in Swedish: all these words mean rock Alint. IX. once called of Delamétherie, and the secondary petrosilex, or néopètre BOOK of Saussure, appear to be one and the same substance,* belonging to the larger quartz agates, and to the secondary rocks; but the true petrosilex, compact and semi-transparent, which is found in Sweden,† in Norway, in Switzerland, and in general in all the primitive rocks. appear to resemble feldspar. This stone forms of itself very considerable mountains, as well in the Alps as in the Uralian chain. In the latter, it is found sparingly in detached blocks and rolled pebbles. Amphibole, though less common Amphibole, than feldspar, holds a no less distinguished rank among the Hornsubstances which compose the primitive rocks. It predo- blende. minates in greenstein; it forms with feldspar the primitive trapps, which differ from basalt; and enters into the composition of syenite, a stone often nearly allied to granite. It also forms of itself very considerable masses. The crystals of European amphibole are most frequently met with incased in substances ejected by volcanos. In Siberia, the traveller Laxman, found, near the lake of Baikal, amphibole crystallized in small prismatic columns, with four large faces, and three small ones. It was hence called baïkalite.§ Mica, a substance remarkable for its metallic brilliancy, Mica. is distinguished from tale by its pure, smooth surface, and by not being like talc unctuous to the touch. The variety of mica which consists of large thin transparent laminæ, is mistermed tale of Muscovy. It is also known under the Latin name of Glacies Mariæ. The Russians, especially in Glass of Muscovy. Siberia, use it in their windows instead of glass; but it soon becomes soiled, and, in some measure loses its trans 122.) Voyages, 1194. Wallerius, Système Minéral, vol. i. p. 283. (Petrosilex æquabilis, Spec. Hauy, iv. 385. Delamétherie, ii. 203. Severgin, Nov. Act. Petropol. 1791. p. 307. Crell, Chim. Ann. 1793. ii. 21. |