of the uncertainty of prosperity and the mutability of human affairs; while many have already derived important advantages from their position as member-candidates.

It is unnecessary further to follow the objects or the merits of the Society. Benevolence is its basis; and, with the numerous associations which have been formed for the relief of those whose age or disappointments in the world have reduced them to a state of poverty and want, this institution holds out a helping hand. Well, then, does it deserve the support of those who are able to render it aid; for we need not refer to individual cases to point out the instability of fortune, even with the most careful, or the uncertainty of life. Would that the mining interest followed the bright example here set forth; and we trust that ere the publication of another volume we shall have to record the establishment of a "benevolent institution for aged and decayed

miners."

ON THE SUPERFICIAL PRODUCTION OF GOLD, OXIDE OF TIN, &c.,

OR ALLUVIAL METALLIFEROUS DEPOSITS.

BY E. HOPKINS, C.E., F.G.S.

Ir is well known that the largest proportion of gold is obtained from rivers and superficial deposits, in granitic districts. The quantity extracted from veins is comparatively small, less pure, and depreciating both in quality and quantity at moderate depths. Gold is always found in its metallic state, almost pure in alluvial deposits, but more or less alloyed when found in veins with other metals and minerals.

Primary rocks, composed of quartz and felspar, with a grey base, approaching to the character of porphyry, saturated with decomposed felspar, containing an excess of potash, with a small proportion of silica in solution, will, by time, produce a metallic efflorescence on the surface, and also fine crystals in fissures, and still more beautiful crystals in vacuities within the rocks. This variety of granite is more subject to form veins than the superficial production by decomposition. Gold and silver, and other metals least subject to oxidation, are commonly seen in a state of superficial efflorescence on the surface, and minute crevices of metalliferous rocks; so identical is the general appearance and the action of this metallic growth to that observed on the face of the negative plate of a battery immersed in a metallic solution, as to

render the analogy complete. The gold obtained from the washings at the foot of auriferous mountains is generally produced in this manner that is, not from veins, but from a superficial efflorescence of the oxidated part of the crystalline mass. I have already alluded in my work on this subject to the slow chemical process which is constantly going on in all moist crystalline rocks, and that this perpetual action in the subterranean base gives rise to disturbances in the rocky structure, and also new production of metals, or rather causing a change from the fluid disseminated state to that of the aggregated crystalline solid state.

These metals, during the act of their new formation, are again subject to become alloyed, or mineralized, by other elements, according to their respective degrees of affinity and local circumstances. Gold and platina are little affected, but tin is subject to oxidation; their respective degrees of purity indicate the character of the mode of production. This process of perpetual change in the mineral kingdom is as constant as the action of the sap in the growth of vegetation. The surface of the auriferous granitics of South America and the Isthmus of Panama, especially those varieties subject to decompose in spherical exfoliation, show this effect in a very remarkable manner. In depth we find the uniform crystalline structure, which, by fine grinding and washing, produces but a mere trace of a yellow tinge of gold; as we ascend, the crystalline aggregation passes, by an almost imperceptible gradation, into a kind of globular structure, like a coarse conglomerate; the respective nucleus of each centre of attraction becomes denser and harder than the parent rock by the concentration of the surrounding silica, like the formation of flints in chalk. Each nucleus becomes enveloped by a series of concentric envelopes, which enclose, as this structural change approaches towards the surface of the rock, an efflorescence of ferruginous mineral, containing gold in minute crystals and in grains. These grey compact silicious centres are called by the native goldwashers in South America the "madres," or mother of the gold, as without these rounded nodules they seldom find sufficient gold in the debris to make it worthy of the labour of washing. The gold, as already noted, is in such a minute state of dissemination in the uniform crystalline base, as to make it almost difficult to detect the mere trace of it; but in the oxidated and decomposed crust the metals become aggregated to different centres, and form into grains, easily detected by the ordinary washing process. The terms mines and gold ores are extremely inappropriate to such deposits.

The stony nuclea, as well as the concentric rings, are completely deprived of their original auriferous contents by this process of internal aggregation. I have many specimens from gold-workings of the above character. After these oxidated

crusts are brought down by the torrents and washed away from the foot, or from the ravines of such mountains, the gold-washers must remove to other localities, or wait a few years until this slow process of Nature supplies them with a fresh crop, as they know to their cost that it would be useless to penetrate into the hard rock in search of the metal, as it must have a surface, vacuity, or a fissure, for its formation. Friable granites are exceedingly productive of alluvial deposits. A great portion of the green carbonates and oxides of copper are produced by a similar process in felspathic rocks. These kind of formations are very prevalent in the south temperate zone, and more common than the veinous sulphurets. The bog iron ore is of the same origin and also the mamillated peroxides of iron deposits, observed as incrustations of the argillaceous series. The gold deposits of California are precisely of the same character, the quantity of the available gold being limited to the amount of the decomposed debris accumulated in the ravines. I know of no primary rocks of the above composition of quartz and felspar, and the friable ferruginous granites, with bright yellow mica, but what contain gold.

Gold and tin, if not embodied, or overpowered, by strong polar substances, such as sulphurets of iron, copper, or other compounds of similar polar tendency, remain in the crystalline base in the disseminated state, they crystallize or effloresce towards the surface. If silicates happen to be in excess, they will produce a fissile structure, and the longitudinal action generated by which causes tension, and, consequently, a series of transverse fractures will take place; the surface of these internal planes becomes gradually coated by the metals the rock may contain, and thus forms veins. But if felspar happens to be the predominating ingredient, and this strongly saturated with iron, or any other substance susceptible of rapid oxidation, or, if the compound be very friable, the exfoliated decomposition will ensue, the free silica aggregate into centres, the felspar is reduced into clay, the metal collects into grains, disintegration takes place, and the thin, loose, friable surface is gradually washed into ravines; the heavier substances fall to the bottom, and accumulate in pools and other parts of the stream presenting the greatest resistance, whilst the lighter particles are washed away by the torrent. Such is the general character of the deposits of the gold and tin stream-washings. These deposits are, therefore, the product of friable metalliferous rocks, and not derived from lodes, the metallic contents of which can only become available by this process of slow decomposition. The value of these deposits depends on the superficial extent of the metalliferous rocks falling towards the head of the ravine, and the level of the plains, to receive the accumulations of centuries. It will be easily conceived, that works of discovery, carried into the hard crystalline base, be

yond the deposit, in such rocks, must prove fruitless; and if such explorations be carried on by men who have no other knowledge to guide them underground than the occasional specks of mineral in the rocks, priany quartz, &c., their reports would be "kindly and promising" until doomsday, and, consequently, an immense capital would be wasted away in unprofitable works, and which has been the case in almost every gold-working carried on by Europeans.

Gold has been often found in the tin-streams of Cornwall, and is frequently associated with the oxides of this metal in the schorlaceous granites, and also in the ravines intersecting the ferrofelspathis rocks of Scotland; in every case it is found accompanied and often attached to the surface of yellow ferruginous quartz. This quartzose compound is easily recognised by those who have studied geology in the works of nature, or those who know the product of rocks from their structure and composition -the true science of geology applied to mines. When the gold was discovered in the streams of Ballin Valley, in the county of Wicklow, in Ireland, almost the whole population of the neighbourhood flocked, like the Californians, to gather so rich a harvest, and actually neglected at the time the produce of their fields. Stream-works were established, and continued for a few years, and the products left a surplus over expenditure, and some large lumps were found. This, however, was soon lost, as well as another capital added to it, owing to the mistaken notion of the existence of a mother lode, whence it was supposed the fine deposit of gold came from.

The best miners of the old school were consulted, and encouraged this idea; the solid mass was soon intersected by numerous trenches, levels, shafts, &c.; every quartz string was driven through, under the impression of finding the grand source of this wealth, but of no avail; with the exception of occasional grains in vacuities, or joints, nothing was found worthy of notice, and the undertaking was abandoned. Yet these same valleys still furnish a small amount of gold annually, and will continue so to do whilst the granitic domes remain subject to decomposition.

The Cornish miner, more especially in the stanniferous districts of Cornwall, is getting now so well acquainted with the character of the crystalline rocks, that he knows from mere appearance of a specimen what kind of granite will produce tin. Schorl appears to be an essential ingredient, or, at all events, a constant associate of the oxide of tin.

Near St. Austell, Dartmoor, and the Land's End, the oxide of tin is in some places so much disseminated through the schorlaceous granite as to render it worth quarrying for the extraction of the tin. All the metals that are found in this disseminated state are, as above noted, always more pure than when they are

found in veins. Those elements which produce joints and fractures, and the formations of veins in the crystalline base, effect the accumulation of the metals in the recesses, at the expense of a considerable amount of alloy of mineralizing substances, such as iron pyrites, the arseniate pyrites, &c.; consequently, metals obtained from veins are never so pure as those procured from the decomposition of metalliferous rocks; when dispersed in the latter, they are comparatively unalloyed. It is important to bear this constantly in mind.

The stanniferous, like the auriferous rocks, are often very friable, and subject to disintegration; the felspar decomposes into clay, according to the character of its component parts, an exfoliated oxidated crust is formed, which is gradually washed down to the ravines, where the several substances are deposited according to their respective gravity-the oxide of tin, being the heaviest, will occcupy the lowest beds in each accumulation. There was a time when miners did fancy that these alluvial deposits of minerals came from lodes, and geologists considered lodes as volcanic productions, but, fortunately for our industrial science, such erroneous notions are now almost matters of history. Some vain attempts have been made to determine the age of this superficial detritus of stream tin, and its associated gravels, but to no purpose; it is like that undefined and improper term, tertiary-it belongs to all ages.

Sometimes we find the tin-stone pebbles, the comminuted quartz grains, and the oxide of tin, left behind on the parent rock, and the decomposed felspar washed away, and this covering becomes so thick as to prevent farther decomposition.

In the Indian Archipelago the same phenomena occur, especially in the island of Banca. This island, with its ridges, conforms in its direction to the Asiatic ranges of mountains, and is composed principally of granite, both ferruginous and schorlaceousthe predominance of the former giving it a general red colour. The oxide of tin is disseminated more or less throughout the schorlaceous granite, and is found in beds at the foot of the granitic range, either immediately under the surface, or at no great distance below it-the greatest accumulations are mostly found in low situations near the schorlaceous ridges. These deposits are composed of clay of various colours, from pure white to yellow and red. The most productive beds of tin ore have been found near the central parts of the peninsula, at the junction of the districts of Sungie-bulu, Klábbet, and Tengá.

The lowest bed is generally of the purest white colour, very light, and adheres strongly to the tongue, like pure clay, on which the ores of tin are deposited in layers differing in richness and extent, according to the locality and the quality of the parent rock. Sometimes the tin is found dispersed through the whole of the bed, commencing immediately under the soil, and