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Petroleum is composed of hydrocarbons, and is found in rocks of all ages, sometimes in subterranean reservoirs of great extent.

Wherever it is found in quantity it is associated with accumulations of gas and salt water, and an essential condition of its occurrence is that a porous bed should exist, capable of absorbing the oil, and that this should be both over- and under-laid by impervious strata, which prevent the oil from escaping.

Pyromorphite will be readily distinguished from copper ores by its high specific gravity ; besides which it is not always green, but often yellowish-green, yellow, or brown. Copper uranite, which crystallises in laminae, exhibits on the larger faces a pearly lustre, and fuses before the blowpipe to a blackish mass. The arseniates and phosphates of copper are soluble in ammonia, and the arseniates give before the blowpipe the characteristic smell of garlic.

Lodes and Crushed Zones are illustrated by Cceur d'Alene, Bunker Hill and Sullivan, and Broken Hill Mines, in all of which the deposits are very large, contain more lead than the disseminated deposits, and the silver contents are considerable. The most troublesome feature about the Broken Hill ores has been the association of zincblende with the galena, more especially because the silver is associated with ea"ch mineral.

It is necessary to divide the silicates into groups according to their chemical composition. Those first dealt with are all silicates of magnesia, and all are hydrous. When sufficiently pure, meerschaum, talc, and steatite, when moistened with a solution of nitrate of cobalt, will give a pink mass which is characteristic of magnesia before the blowpipe.

The first three minerals in the table are sufficiently soft to be scratched by the nail ; but serpentine is harder, approximating in hardness to calcite.

The occurrence and distribution of gold, &c., in alluvial deposits has formed the subject matter of another chapter, but a few remarks may be added on deep alluvial deposits.

It is a well-known fact in all mining districts that the junctions of lodes are generally the richest points, always supposing that the junction takes place in " kindly country " ; the explanation of this is simple on the aqueous theory of filling of lodes.

There are many varieties of quartz which claim some attention as ornamental stones, all of which consist of silica.

Rock Crystal occurs crystallised in six-sided prisms with pyramidal ends ; it is perfectly clear and transparent, and is used both for optical instruments and for ornamental purposes. It is sometimes found in crystals of enormous size, several weighing from 8 to 10 cwts. having been recorded; and it is reported that about a century ago a drusy cavity was opened at Zinken from which 50 tons of rock crystal were obtained, which realised 60,000.

THE determination of the more important minerals which may be met with deserves special attention at the outset of the subject, and the present chapter will be devoted to the simpler means of distinguishing them. It is not, however, intended to encroach upon the detailed study of mineralogy, on which subject the many valuable treatises published can be consulted.

REFERRING to Chapter VI., it will be found that lodes traverse the country in many different directions, and that, in districts which have been carefully studied, those lodes which follow different courses are, as a rule, characterised by special minerals.

All ores of lead give a bead of metallic lead when heated on charcoal with soda before the blowpipe. They will be readily distinguished one from the other by the characters given in the following table :

Lead mining in Europe is inseparable from silver mining, as silver is mostly extracted from argentiferous galena ; cheap labour and scientific appliances enabling poor ores, containing only 9 or 10 ozs. of silver per ton, to be treated at a profit, both lead and silver being extracted.