genus of siliceous earths very common in Europe. According to Kirwan, the quartz are in general the purest of the siliceous kind, though most of them contain a slight mixture of other earths: the most obvious distinction among them arises from their opacity or transparency. Cronstedt gives the following characteristics of it: 1. It is generally cracked throughout, even in the rock itself, whereby, as well as by its own nature, it breaks into irregular and sharp fragments. 2. It cannot be easily made red hot, without cracking still more. 3. It never decays in the air. 4. Melted with fixed alkali in a due proportion, it gives a more solid and fixed glass than any of the other siliceous stones. 5. When there has been no interruption in its natural accretion, it always crystallizes into hexagonal prisms pointed at both ends. 6. It is met with in clefts, fissures, and small veins in rocks; it seldom forms large veins, and still more rarely whole mountains, without a mixture of heterogeneous substances. It is found,

1. Pure, of several varieties, as, (1.) Solid, or having no visible particles, and called fat quartz. This is either transparent, white, blue, or violet-coloured. The first kind is met with in the copper-mines in the northern part of Norway and Siberia, and has no regular form, but is as clear as the finest crystallized quartz, or rock crystal. (2.) Grained quartz, of a white or pale green colour, found in various places in Sweden. (3.) The sparry quartz, which is the scarcest of the whole, and ought not to be confounded with the white felt-spar, because it is of a smoother appearance, and breaks into larger and more irregular planes. It is found of a whitish yellow, from the gold mines in Hungary; or white, from the island of Uto. Brunniich tells us, that the Hungarian gold and silver mines near Hodentch, which have veins frequently some fathoms wide, afford a kind of lamellated and porous quartz. It is met with of white, yellow, and blue colours, and it is sometimes finely crystallized in pyramidal figures.

2. Crystalized quartz, or rock crystal. See Crystal.

3. Impure quartz. Of this there are two kinds, (1.) Mixed with iron, in form of a black calx. It is black, gloomy, and contains a great quantity of iron. It is found in Sweden. (2.) Mixed with copper, and of a red colour, found in the same country.

Cronstedt observes, that quartz in general, and especially its crystals, are very commonly supposed, when yet in their soft and dissolved state, to have included within them some vegetables, for instance grats and mosses. "This (says he) I cannot absolutely deny; but it deserves carefully to be examined if that which is shown as a grat be not an asbestos, or a striated cockle; and the moss only branched varieties filled with earth, which, by their being ramose, bear a vegetable appearance. It is very common in agates, and makes them of less value than they otherwise would be. This is most generally the case with those stones which are shown as including vegetables; and, for my own part, I have..." Quartz have never been so fortunate as to meet with any othera."

M. Magellan remarks, that quartz is one of the principal kinds of stone which contain metals. Some of the Hungarian veins consist entirely of it, and the gold is so minutely dispersed, that it cannot be discerned by the best microscopes before it is separated by pounding and washing. The width of the veins, some of which are half a fathom, and some still more, repay the trouble and expenses, which the small quantity of gold would not otherwise counterbalance. Nature has not anywhere produced mountains of pure quartz; for though some rocks in Sweden are ranked among the quartzes, they are undoubtedly mixed with heterogeneous matters. Near Lauterberg upon the Hartz are veins of this stone from one to three fathoms wide, consisting of a loose sand, in which they find the copper ore in nests. In the Danish isle of Anhalt we meet with triangular quartz pebbles. There are likewise crystals of quartz having water inclosed in them; some fine pieces of this kind are to be met with in the Imperial cabinet at Vienna, &c.

Rock crystals are generally found upon or among quartz, and are to be met with in all parts of the world. The greatest number are furnished to the European countries from Mount Saint Gotthard in Switzerland.—Here large pieces, weighing from 5 to 800 pounds, were found at Grimmelberg; one of 1200 pounds was found some years ago at Filibach in the Wallais; and a piece six feet long, four broad, and equally thick, was found in the island of Madagascar, a place where these natural productions are of the most extraordinary size and perfection.

When great quantities of quartz are continually agitated by the sea or river water, they are sometimes reduced to such very minute parts as to be easily carried away, suspended in the water; and there are sands of so minute a size as to measure less than the two or three hundredth part of an inch. These are called quick-sands. Immense tracts of land consist only of loose sands, particularly along the sea-shore in many parts of Europe. Some suppose that sea-water has the power of producing this sand out of its own substance; and their surfaces, in general, are polished, as to show that they could not be reduced in size by rubbing against each other; but we know not as yet that such a production has ever been demonstrated. When sand is about as big as peas, it is called gravel; and when it is free from saline and heterogeneous particles, it is employed in making mortar, and other economical purposes. That which is very pure serves for making flint-glass, with red calces of lead, and the proper alkaline flux; but when mixed with ferruginous black sand, the glass assumes a greenish black colour. "This (says M. Magellan) I have seen among the various specimens of glass made by Mr E. Delaval, F. R. S. who produced a very fine transparent and colourless glass out of the same sand with which he had made some of that black glass, and this only by separating from it all the ferruginous mixture."