be the repositories of other kinds of ore. As the main veins or "lodes" are thus intersected by others of more recent date, they will also be more or less displaced; and the ascertaining of these facts is ever of the highest moment to the mining industry of a district, to say nothing of its importance to correct geological deduction. The mapping of these directions and the colouring of the primary and secondary veins according to the kind of ores they contain, is a work to be done by the mining-engineer for each respective district; though it is now well known, that according to the rocks of a country and the age and direction of its hills, so also are the general strikes of the veins and the nature of their metalliferous contents. The direction of veins and the nature of their contents reveal a chronology and order within their respective areas as much as the stratified systems do, and a knowledge of these relations is alike of scientific and economic importance.

At the present time attention has been less directed to the direction of veins and the corresponding nature of their contents, than to the modes of their formation and their subsequent infiltrations. At one time igneous action was called in by hypothesists, not only to produce the original fissures, but to accomplish also the subsequent fillings-in ; and, in fact, to perform the most opposite and contradictory functions. Now, however, while it is agreed that the rents and fissures were originally produced by, and owe their linear directions to, vulcanic commotions, it is the general and growing opinion that their subsequent fillings-in with sparry and metallic matters are due to the infiltration and deposition of chemical solutions. Heated waters and vapours, or hydro-thermal action, as it has been termed, has been the main agent in dissolving, carrying upwards, and redepositing the mineral and metallic matters—that is, the sparry vein-stuffs as well as the crystallised ores. In fact,

similar spars and ores can be produced in the laboratory by chemical and electrical means, and all the more certainly that heat is present to facilitate the operation. What can

be simulated on the small scale by art we may readily believe to be producible by nature on the large, and thus to chemical and electro-chemical agency are now generally attributed the formation of vein-stuffs and their associated ores. We have spoken of the greater efficiency of heated waters in dissolving and holding in solution mineral and metallic matters, but the effects of water in general (whether hot or cold) must not be overlooked, it being the grand medium through which the contents of veins have been conveyed to their present positions. It is this belief in chemical solution and redeposition which distinguishes the modern theories of mineral veins from the older views of sublimation through igneous or plutonic action.

Whatever may have been the immediate agent of infiltration and deposition, one thing is certain, that veins, and especially metalliferous ones, are most abundant in areas that have been long subjected to igneous action. There is the closest connection, and necessarily so, between the two phenomena, as it is only in convulsed districts that rents and fissures can occur, and in such districts also that such fissures have the greatest chance of being most rapidly filled with mineral and metallic precipitates. Wherever, therefore, there are old hill-ranges and primary areas that have been repeatedly subjected to subterranean forces, there we may expect veins and cross-veins-each set representing a long course of time, and being for the most part filled with its own special ores and vein-stuffs. All portions of a hill-range may not be rich alike, for there are certain centres in which the producing forces seem to have acted with greater intensity, or at all events to have been longest.

continued, and it is in these that the greatest variety of veins and cross-veins occur, and from these also that the greatest variety of metallic ores are to be obtained. What the law that has determined the greater richness of certain districts, science cannot as yet give the slightest indication, any more than it can tell why certain areas that were once convulsed with igneous activity have long since been cold and silent. All that can be done in the mean time is simply to note the facts; and these, when correctly recorded, become of the greatest importance to industrial operations, as they will one day or other do to scientific deduction.

The importance of correct information on all that relates to metalliferous veins and deposits cannot be too highly valued, and especially in countries like Britain, that depend so much upon the metals for their mechanical, manufacturing, and commercial greatness. Whether as a medium of exchange, for the fabrication of implements and the construction of machines, or merely for objects of luxury and ornament, the metals are all-important; and whatever tends to certainty and facility in obtaining their ores is deserving of a nation's encouragement. Without the metals there cannot, indeed, be high and substantial progress in civilisation; and in modern times a nation's place may be safely indicated by the facilities she has of obtaining them. Her hills may be bleak and barren, and little fitted for the amenities of agriculture; yet beneath that poor and rugged surface may lie mines of untold wealth, and the readiest means of manufacturing and commercial greatAnd such is usually the contrast that presents itself in mining and metalliferous districts. Cold and retentive clays, ungenial moorlands and uplands, are too often characteristic of coal-tracts, as witness those of Northumberland, Durham, and Lanarkshire; while cliffs, and scars, and bleak unapproachable ridges, are the common concomitants

ness.

of metalliferous regions, as those of Derbyshire, Wales, and Cornwall.

The great value of primary districts lies, as already mentioned, in their metalliferous lodes and veins, or in the stream-drifts that have been weathered and worn in course of ages from the cliffs and precipices above. The vein lies in the solid rock, and must be mined with great labour and outlay; the stream-drift, on the other hand, is but the water-borne debris from the veins above, and demands merely sorting and washing. The stream-work is the ready and primitive method of obtaining the ores and metals; the mine is the laborious but more certain appliance of modern times and modern requirements. In conducting a stream-work, little more is needed than manual labour and care; in managing a mine, mechanical appliances, engineering skill, and correct geological deduction are indispensable at every stage of the undertaking.

Such is a brief, and necessarily sketchy, outline of the nature and origin of veins and vein-stuffs. The rents or fissures originally produced by subterranean convulsion are subsequently filled by infiltrations of mineral and metallic matter, and thence become the veins which seem to ramify and reticulate through the earth's crust like the veins through vegetable and animal structures. A fissure may be produced in an instant by earthquake convulsion, but ages may pass before it be completely filled by sparry minerals and metallic ores-the slow depositions from aqueous percolation and solution. As water is ever percolating the earth's crust, so it is ever dissolving from one part and redepositing in another; and this power of dissolving is no doubt greatly augmented by heat, just as rapidity of precipitation and crystallisation may be facilitated by electromagnetic currents which are incessantly traversing the

rocky framework. In this way the fissure becomes a vein; and as each set of veins has a fixed direction, and is charged with its own peculiar vein-stuff and metal, the ascertaining of these directions and peculiarities is at once of the highest scientific and economic importance. As the original fissures are produced by volcanic convulsions, and the subsequent fillings-in by slow and gradual precipitation from solution, veins will occur most abundantly, of course, in districts that have been longest subjected to those agents; and such tracts are necessarily those occupied by primary and transition strata. In these districts, bleak and barren and inhospitable, the mining industry of the world is chiefly situated, their subterranean wealth compensating, and often more than compensating, for their want of agricultural fertility and amenity. As the metals are indispensable to mechanical, manufacturing, and commercial progress, so they are generally regarded as powerful auxiliaries of civilisation; hence the importance of all that attaches, scientifically and industrially, to their history, their modes of occurrence, the means of obtaining them, and the process of reducing them from their ores and associated vein-stuffs.