All the substances of which the ordi nary metals form the basis, are not equally abundant in nature ; a great pro portion of the numerous mineral species which figure in our classifications, are mere varieties scattered up and down in the cavities of the great masses or lodes. The workable ores are few in number, being mostly sulphurets, some oxides, and carbonates. These oi.casionally form of themselves very large masses, but more frequently they a:e blended with lumps of quartz, feldspar, and carbonate of lime, which form the main body of the deposit ; as happens always in pro-. per lodes. The ores in that case are ar ranged in small layers parallel to the strata of the formation, or in small veins which traverse the reek in all directions, or in nests or concretions stationed ir regularly, or finally disseminated in hard ly visible particles. These deposites sometimes contain apparently only one species of ore, sometimes several, which must be mined together, as they seem to be of contemporaneous formation ; whilst, in other cases, they are separable, hav ing been probably formed at different epochs.
The following general observations on the localities of ores, and on the indica tions of metallic mines, show their dis positions in various geological forma tions.
1. Tin exists in primitive i rocks, appearing either in interlaced masses, in beds, or as a constituent part of the rock itself, and more rarely in dis tinct veins. Tin ore is found indeed. sometimes in alluvial land, filling up low situations between lofty mountains.
2. Gold occurs either in beds or in veins, frequently in primitive rocks ; though in other formations, and particu larly in alluvial earth, it is also found. When this metal exists in the bosom of primitive rocks, it is particularly in schists ; it is not found in serpentine, but it is met with in graywacke in Tran sylvania. The gold of alluvial districts, called gold of washing or transport, oc curs, as well as alluvial tin, among the debris of the more ancient rocks.
3. Silver is found particularly in veins and beds, in primitive and transition for mations ; though some veins of this metal occur it secondary strata. The rocks richest in it are gneiss, mice-slate, clay-slate, graywacke, and old alpine limestone. Localities of silver-ore itself are not numerous, at least in Europe, among secondary &mations ; but it oc curs in combination with the ores of cop per or of lead.
4. Copper exists in the three mineral epoehas ; first, in primitive rocks, prin in the state of pyritous copper, in beds, in masses, or in veins ; second, in transition districts, sometimes in masses, sometimes in veins of copper pyrites ; third, in secondary strata, especially in beds of cupreous schist.
5. Lead occurs also in each of the three mineral epochas ; abounding par ticularly in primitive and transition grounds, where it usually constitutes veins, and occasionally beds of sulphu ratted lead (galena). The same ore is
found in strata or in veins among secon dary rocks, associated now and then with i ochreous iron-oxide and calamine (car bonate of zinc) ; and it is sometimes dis seininated in grains through more recent strata.
6. Iron is met with in four different mineral eras, but in different ores. Among primitive rocks, magnetic iron ore and specular iron ore occur chiefly in beds, sometimes of enormous size ; the ores of red or brown oxide of iron (hema tite) are found generally in veins, or oc casionally in masses with sparry iron, both in primitive and transition rocks ; as also sometimes in secondary strata ; but more frequently in the coal-measure strata, as beds of clay-ironstone, of glob ular iron, oxide, and carbonate of iron. In alluvial districts we find a cross clay iron stone, granular iron-ore, bog-ore, swamp-ore, and meadow-ore. The iron ores which belong to the primitive period have almost always the metallic aspect, with a richness amounting even to 80 per cent. of iron, while the ores in the posterior formations become in general more and more earthy, down to those in alluvial soils, some of which present the appearance of a common stone, and afford not more than 20 per cent. of metal, though its quality is often excellent.
7. Mercury occurs principally among secondary strata, in disseminated masses, along with combustible substances ; though the metal is met with occasion ally in primitive'countries.
8. Cobalt belongs to the three mineral epochal ; its most abundant deposits are veins in primitive rocks ; small veins containing this metal are found, however, in secondary strata.
9. Antimony occurs in veins or beds among primitive transition rocks.
10, 11. Bismuth and nickel do not ap pear to constitute the predominating substance of any mineral deposits ; but they often accompany cobalt.
12. Zinc occurs in the three several formations : namely, as sulphuret or particularly in primitive and transition rocks ; as calamine, in secon dary strata, usually along with oxide of iron, and sometimes with sniphuret of lead.
An acquaintance with the general re sults collected and classified by geology mast be our first guide in the investiga tion of mines. This enables the obser ver to judge whether any particular diss trict should, from the nature and arrange ment of its rocks, be susceptible of in cluding within its bosom, beds of work able ores ; it indicates also, to a certain degree, what substances may probably be met with in a given series of rocks, and what locality these substances will pre ferably effect. For want of a knowledge of these facts, many persons have gone blindly into researches equally absurd and ruinous.