have their greatest length in an easterly and westerly direction."* The section on Fig. 1 shows the general arrangement of the rocks in the neighbourhood of Rio Tinto.

FIG. 1. GENERAL SECTION AT RIO TINTO.*

1. Porphyry. 2. Clay-slate, &c. 3. Pyrites with overburden of oxide of iron

Except in size, the mineral masses differ but little from one another, and the characters common to all are as follows:-The ore is granular iron-pyrites, with a very small mixture of copperpyrites, and about four per cent. of silica: here and there are slight traces of other ores, of copper, lead, and zinc. The ore, instead of being distributed, as is the case in most mineral veins, in strings, layers, or bunches, among unproductive "veinstuff," forms a homogeneous mass, unmixed with any foreign matter, or traversed only by a few insignificant veins of quartz. In the larger deposits "riders of rock do sometimes occur, splitting up the mass, more or less, into several subdivisions; but these are as distinctly marked off from the body of the ore itself as the rock of the surrounding country. The horizontal section is rudely lenticular in shape, its longer axis ranging parallel to the strike of the slates in the neighbourhood. No bottom has been reached in the larger deposits, but some of the smaller masses have been followed downwards till they thinned away altogether.

These masses of ore are surrounded by a belt of altered slate, known to the miners as the "Salbanda," from the German "Sahlband," consisting in the main of bleached and porcelanized rock: the part, however, immediately in contact with the ore is often soft and crumbly, seemingly from the effect of chemical action occurring at the junction.

The ore itself never shows at the surface, but is covered by a gossan" or "overburden," consisting in the main of oxide of iron, mixed with red clay and fragments of the adjoining "country," slate, or porphyry, as the case may be. There can be little doubt that this is the result of atmospheric decomposition of the pyrites: at Rio Tinto lumps of it have been found, containing unaltered pyrites in the centre. The overburden varies in thickness at different mines,

*

*Spanish report on Rio Tinto, where will also be found a detailed account of the lithology of the district.

being very rarely as little as 20, and sometimes as much as 160 feet: on an average perhaps about 40 feet.

The ground covered by the gossan is usually lower than the level of the surrounding country, and looks as if it had sunk bodily, while the Sahlband stands up as a wall all round: the cause of this depression seems to be unequal atmospheric denudation, caused by the unequal hardness of the crumbly oxide and the firm Sahlband. A fanciful notion among the Spanish miners is, that the ore boiled up in a melted state from below till it reached the surface, and then shrank in cooling.

The Sahlband and the depressed crust of oxide of iron form sure guides by which we can detect, without underground explorations, the presence of a pyritous mass, and determine its shape and size. Some caution, however, is needed, for other deposits of oxide of iron occur, which to the unpractised eye are very like those which overlie a mineral mass, but beneath which no ore will be found. These may be called "false caps" of oxide of iron. Many of these have doubtless been formed by water charged with oxide of iron, produced by the decomposition of pyrites, for like deposits, the recent origin of which is proved by their containing bits of slag, are now being laid down by the water issuing from the mines. Some of these "false caps," however, are found in places where no stream can now flow: in this case they may have been deposited when the surface configuration of the country was different from what it is now; or they may be the remnants of a pre-existing pyritous mass, the greater part of which has been denuded away, and the remnant entirely oxidized. If the latter explanation be correct, extra precautions will be necessary when exploring for a deposit of pyrites, for while surface indications give the position and horizontal section, boring will be required to determine whether any unaltered mineral remains below the oxidized crust.

The general look of the country is tame and monotonous: after a time, however, the constant repetition of the same features begins to impress forcibly on the mind their very peculiar character. As far as the eye can reach, there stretches what looks like an unbroken flat, thickly overgrown with gum-cistus. Entering on this seeming plain, we find that it is deeply channelled in every direction by steep-sided brook and river valleys; but till we actually stand on the edge of one of these valleys there is scarce anything to lead us to suspect their existence: looking back, we wonder what has become of those precipitous glens down which our horse so carefully planted each footstep, and up which he so laboriously toiled, for the country looks an unbroken flat; ahead it is to all appearance the same, though closer acquaintance will show us how deceitful looks are. With scenery so marked daily before the eye, a conviction is very forcibly brought home to the mind that the country was once

just such a plain as it now seems to be, and that the valleys were carved out afterwards, and all weariness at the monotony of the landscape vanishes in the delight of realizing the idea of a Plain of Marine Denudation, not by the aid of maps, sections, and description, but the thing itself, the size of nature, yet as clearly shown as in a model.

We may now turn to a few of the mines at work, beginning with that of Santá Domingo,* in the province of Alemtejo in Portugal, not far from the Guadiana and the Spanish frontier.

The deposit, which is of the usual lenticular shape, but irregular in outline, is between 500 and 600 mètres long and 70 mètres across at the widest part, and is split up by several "horses" or "riders" of slate: the ore is uncrystallized iron-pyrites of a dull grey colour, intimately mixed with a small percentage, from 2 to 4, of copper-pyrites; the latter sometimes occurs in larger quantity, and hard specimens may be picked out containing perhaps 20 per cent. of copper. Galena and other minerals are also sparingly present, and the joints are thinly coated with carbonate of copper.

FIG. 2. SECTION ACROSS THE ADIT AT SANTA DOMINGO.

Fig. 2 is a section across the adit. The plane of separation between the mineral and the sahlband was most distinctly marked and haded to the north at an angle of from 45° to 80°: it was marked at some spots by beautifully polished slickenside. The sahlband itself consisted of a belt (2), next the ore, of a white, soft, quartzose rock, about 2 or 3 mètres broad; beyond this is a belt, said to be as much as 30 mètres broad, of very hard, flinty, porcelanized rock (3), white with narrow red stains: this passes outwards into slate rock. Here and there in the outer belt is seen a breccia of porcelanized rock and white quartz cemented by oxide of

I gladly take this opportunity of acknowledging the courtesy of Mr. Mason, the managing partner, who entertained us during our stay.

iron. The altered rock seems to be the result of intense heat acting under pressure; the breccia and slickenside point to movements and rending of the rocks, the fragments being afterwards cemented by water percolating from the ore through the resulting fissures. The soft belt is probably only a portion of the porcelanized rock corroded by decomposing agents produced by chemical action at the junction with the ore: such action is constantly going on, for the roofs and walls of the mine are thickly coated with incrustations and stalactites of sulphate of copper, and the water that flows from the workings holds in solution much of this and other salts.

The mine is entered by inclined adits, and where these strike the ore, a level is carried in the sahlband of the southern or foot-wall alongside the mineral mass: the overhanging of the solid mineral, which to a certain extent protects the tunnel, makes this side the best for driving. At right angles to the level, galleries or "cross cuts," about 40 feet high, are driven across the mass to the northern wall, ribs or pillars of ore being left between to support the roof. The ore is carried by boys in baskets, made of esparto grass, into the tunnel, drawn up to day in waggons, and conveyed on a railway, with startling curves and gradients, to the shipping-port on the Guadiana.

Thus much of the mine itself: but the spot has an interest for others besides the miner and geologist. In the middle of this wild country, on a spot which but a few years ago was as lone and barren as all around it is still, there has sprung up a good-sized village, with the offices, stables, and outbuildings of the mine, a church, and the handsome house of the resident partner: weekly markets are held, and the place boasts its clergyman, doctor, and police. All this change has been brought about by the energy of a single man, to whom and to whose like the country owes no small debt of gratitude. For this material civilization cannot fail to lead some day to higher results. It will be envied and sought after by the people of the land, and will be found to be beyond reach so long as their present ignorance lasts. To obtain it, education in the highest sense is not needed, but some degree of knowledge is; and even if this be of the most technical kind, and have only the lowest ends in view, it cannot fail to rouse and strengthen the mind; and men, taught to think for themselves on one class of subjects, will soon shake off the fetters of dogmatism, and do the same on all; and thus by a side-wind we may hope that education proper will find its way into a land where any attempt to introduce it directly would be put down as profane, or revolutionary, or both. Undertakings such as this, whether they are started with such a view or not, are true missionary work, and almost the only kind of missionary work practicable under the circumstances.

Of the large mineral deposits, that of the Tharsis comes next in

geographical order. A plan is given in Plate II.,* from which it will be seen that there are four large masses of ore, all of which are clearly pointed out by surface indications, and have been proved by shafts and borings, though mining has been carried out only in one. The "gossan" reaches here at some spots the great thickness of 130 feet, and is believed to average 110 feet. The ore was at one time extracted by underground workings; but now, in spite of the difficulties attending the removal of the vast "head" of unproductive matter, it is entirely raised in "opencast," and the mine is really a large quarry. The property has lately passed into the hands of a Scotch firm, and is being energetically opened out.

The mine of the Coronada, about three leagues west of the Tharsis, has been worked in bygone times, and one of the old adits has lately been cleared out by modern adventurers: it shows well the nature of the work of the "old man," running up and down hill, and winding in a tortuous way, seemingly for the purpose of avoiding the harder portions of the rock, which could not be cut through without blasting, and round or over which the crafty miner worked his way, choosing soft ground even at the risk of extra driving. The remains of an old wooden wheel used for unwatering the mine were also found; it seemed to have worked on the principle of a dredging-machine, being fitted with buckets, which dipped into and raised the water, till, after passing the highest point, they became inverted, and discharged their contents into a gutter. A like contrivance is now, or was lately, in use at the Tharsis.

We may now turn to the Buitron Mine, near the town of Valverde, of which a sketch-plan is given in Fig. 3.

The sett contains two deposits, the eastern one alone being at present opened out. The ore is reached by an adit as at Santa Domingo, but owing to a very deep valley it has been possible to

* Reduced from a plan kindly lent me by Mr. Thornthwaite.