Masses, or " pipe-veins," as they are often termed by the miner, are not so easily defined : the beat idea which can be given of them is that of an irregular branching cavity descending either vertically or obliquely into the rock, and filled up with metalliferous matter. Deposits of this nature are less common than the two former classes; they usually con tain either copper or lead, and some of the rich oxides of iron appear to belong to a similar formation.
Fragmentary Deposits occur associated with and indeed forming part of many of the loose superficial beds of sand and gravel which occur in the valleys of mineral districts, consisting of the detritus of the neigh bouring mountains, which has been washed down from thence at remote geographical epochs. The mineral substances found in these deposits, which may be considered as having originally been derived from veins or beds in the vicinity, are not, in most cases, mixed up indiscriminately with the alluvial matter, their greater specific gravity having occasioned them to be deposited in distinct layers by themselves, usually towards the bottom of the mass. Tin and gold are the metals which most commonly occur in deposits of this kind.
Geological Position.—The above-mentioned mineral and metalliferous deposits are not found promiscuously distributed throughout all rocks or soils; on the contrary, there are certain rocks, or rather assem blages of rocks, to which they may be considered as in a great measure peculiar. Granite, porphyry, and the older igneous rocks, generally are metalliferous, and often eminently so ; but mineral deposits are on the whole most abundant in rocks of sedimentary origin, and more especially in and near situations where these two classes of rocks (the igneousand scdimentary)are in contact, or where a metamorphic struc ture has, from the action of internal causes, been superinduced upon the latter. It does not appear that nature has confined particular metals to any exclusive kind of rock, yet traces of a general association may still be perceived. Thus, tin, copper, gold, silver, and certain deposits of iron, are most abundant in the class of rocks usually termed primary ; while lead, zinc, mercury, and the earthy ores of iron, are most abundant in the older secondary rocks, which also contain our principal deposits of rock-salt. Veins are of most common occurrence
in igneous and primary or metamorphic rocks, in the vicinity of which the derivative fragmentary deposits are therefore most often found. Mineral-beds, although sometimes occurring in primary rocks, are most common in secondary countries ; and irregular masses, or pipe-veins, are most strikingly developed in limestone districts, where they probably occupy original cavities in the rock.
Metallic Ores and Mineralising Substances.—The state in which the metals are found, the full consideration of which belongs to chemistry and mineralogy, will next require our attention, so far as it is immedi ately connected with the business of the miner. It must be well known that the metals are very rarely presented to us in a pure or metallic state, although native masses of copper and iron are occasionally met with; but gold, from its small affinity for oxygen and other mineralis ing substances, although frequently alloyed, is never mineralised. The great bulk of the metals then, with the exception of gold, arc found in the state of ores, that is, chemically combined with certain mineralising substances, which completely disguise, and, in fact, till separated by metallurgical processes, destroy their usually recognised and useful properties. The moat important of these mineralising bodies are oxy gen and sulphur ; the next in rank are chlorine, and the sulphuric, carbonic, and phosphoric acids. The mode in which they combine with the metals is in binary compounds. Of the former we have examples in iron, lead, and mercury, which, when mineralised by sul phur, form respectively the following sulphurets, iron pyrites, galena, and cinnabar : we observe the latter in all cases where the metals are mineralised by acids, as in spathose iron-ore, or carbonate of iron, in which one binary compound, the oxide of iron, is united to another element, the carbonic acid. From this naturally compound state, in which the metals almost invariably occur, arises the art of metallurgy, which, although generally considered totally distinct from that of mining, is nevertheless most intimately connected with it.