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Native silver often occurs accompanying other silver ores, and is sometimes sufficiently abundant to form its most valuable constituent, as at Kongsberg, in Sweden, and in Peru. Argentite or silver glance, which is the sulphide of silver, is perhaps the most important of the ores of this class ; but the antimonial silver ores also occur in considerable abundance in certain localities, notably in some of the American mines.

This beautiful stone is blue ; opaque or semi-translucent ; and is often traversed by veins of pyrites. It is a very complex mineral chemically (if, indeed, it must not be regarded as a rock), consisting of silicate of alumina, with soda, lime, sulphur, chlorine, <kc. So long as the pigment which bears its name was obtained solely from this source, the price of the colour was enormous ; but since it has been manufactured artificially the price has been greatly reduced, and ultramarine can now be obtained at a fraction of the price formerly paid for it.

Special attention has been devoted to a description of the Australasian alluvial deposits, because they illustrate nearly every condition which can prevail ; but it may be well to allude to British Columbia and California as affording illustrations on a gigantic scale and exhibiting features which are perhaps better studied there than elsewhere. This mountainous country affords evidence of the important part glacial action has played in shaping its ranges and forming its lakes.

The present scheme will be found to answer in most cases for the identification of minerals of common occurrence, but will not discriminate between the many rarer species. Fully a thousand minerals have been described from time to time, but in this book it is not proposed to deal with more than about one hundred which are of common occurrence. In any case, however, the system adopted will give some clue as to the nature of an ore.

It must be borne in mind that the prospector has not only to find mineral which is in itself intrinsically valuable, but must also know enough regarding the methods of working, the costs of different processes, fec., to determine whether the metal or mineral is present in sufficient quantity to make the deposit of value.

The stone should be sampled every few feet and taken from wall to wall in order to arrive at a fair estimate of its value.

Cinnabar or sulphide of mercury is the only regular and valuable ore of this metal. It is of a bright red to brownish-black colour, is always red in powder, and affords fumes of quicksilver when heated with soda on charcoal. Native mercury and amalgam also occur. Some grey copper or tetrahedrite yields mercury.

These surface deposits must necessarily exist in the vicinity only of the reefs or lodes from which they have been shed, and their occurrence affords considerable inducement to prospect for reefs in the immediate neighbourhood ; but the gold which is found in rivers and streams does not necessarily point to the close proximity of the reefs from which it was derived ; still less does the occurrence of alluvial gold in buried river beds indicate the proximity of reefs.

The mechanically formed sedimentary rocks consist of mud, clay, sand, and gravel, together with their corresponding shales, slates, sandstones, and conglomerates, which have been produced by consolidation of the sediments. They are also at times changed, by a process of metamorphism, analogous to what takes place in the formation of granite, to schists, gneiss, or quartzite.

A class of reefs not hitherto described in these pages, which are called " saddle reefs," occur at Sandhurst, Victoria. Fig. 34 gives an idea of their shape, and also suggests that they may have been formed at the intersection of a parallel system of fractures with cross-joints in the rocks ; they may, however, be due to foldings in the strata.