arctic clays do not occur, and they must be classified in the postglacial series. An elevation of at least 800 feet therefore has taken place in this district, since the retreat of the arctic mollusca to their present latitude.

There are also signs in Scotland, of elevation of the land since the glacial epoch. The large bed of Ostrea edulis, which extends through the whole plain at Stirling, and is associated with a clay containing several specimens of the whale, is post-glacial, and the shells are undoubtedly in situ. In the course of the river Irvine, the following section occurs:-(1) clay, with Cyprina islandica; (2) sand, with remains of whale; (3) purely littoral sand both in aspect and contents, equivalent to that on the present shore. The gradual upheaval of the land in post-glacial times, converting the habitat of Cyprina islandica over which the whale journeyed into the shallow sandy shore which became its tomb, is by this section very closely indicated.

The courses of arctic currents and of currents equivalent to the Gulf-stream, must have been greatly affected by the physical alterations in the relative position of land and water, produced during the history now briefly sketched, and have acted upon the direction of the isothermal lines.

In certain beds, the fauna appear to indicate results of this description. At Bisæt, near Christiania, Isocardia cor, a shell which inhabits the Mediterranean, as well as the southern parts of the Scandinavian waters, is associated with the eminently arctic species Tellina calcarea. A corresponding fact may perhaps be quoted from the Hebrides at the present day, where several especially northern forms reach their most southern limit; and certain peculiar species have no locality recorded between that district and the Mediterranean.* A fossil bed at half-tide mark, in the Kyles of Bute and other localities, differs somewhat in its contents from the present fauna of the district. Pecten maximus and Ostrea edulis are far larger than now found in the Firth, while Psammobia Ferroensis and Tellina incarnata are more abundant than in recent dredgings in the neighbourhood.

Reviewing the various points indicated in the present paper, we arrive at the following suggestions for further investigation:

(1) The course of physical changes from the glacial epoch to the present day, was the same in its broad outlines in Norway and Scotland.

(2) These changes were gradual and have left their evidence in the shell-beds as well as in physical phenomena.

(3) It is necessary, therefore, to separate and classify these various shell-beds and not include them under the general names of "Drift" and "Raised Beach."

*See Reports by Mr. Gwyn Jeffreys on Dredging among the Hebrides.

(4) A general order of succession and variation in the glacial deposits characteristically prevails both in Scottish and Norwegian localities, and embraces the phenomena of an epoch, rather than the merely subordinate accidents of local circumstance.

III. ON THE IRON-PYRITES MINES OF ANDALUCIA.

By A. H. GREEN, M.A., F.G.S., of the Geological Survey of England and Wales.

FOR abundance and variety of mineral products, scarce any land in the world can match Spain; but owing to the state of torpor into which this once active country has fallen, it has become all but a matter of impossibility to work her mines with profit. English and French capital and energy have, however, of late, in many instances, successfully battled with the difficulties of such an undertaking; and among the districts thus opened out is a mineral tract reaching across the western part of Andalucia and the adjoining portion of Portugal, which contains many very large and remarkable deposits of Iron-pyrites.

My own knowledge of this ground was gathered during rather a hasty visit, but I have been able to add to it from the works quoted below: these, however, are but little known to general readers, and are besides of a somewhat technical character, and I therefore hope that the rather more popular account which I am undertaking may be neither unacceptable nor superfluous.

The rock of the district is clay slate, Silurian in age,† bedded, but not cleaved; talcose and micaceous slates are also met with, and here and there beds of quartzite. The strata rear up at very high angles, and are much contorted; but I was told that a general northeasterly and south-westerly strike could be traced. Over the slate tract are scattered many masses of porphyry of different kinds, passing here and there into diorites, accompanied at some points by masses of cupriferous iron-pyrites, which at the surface are represented by deposits of oxide of iron, known in the country by the names of colorados, monteras de hierro, or requemones. The porphyritic masses are as a rule not very large, of small breadth, and

*Notes on the Copper Mining Districts of the Provinces of Seville and Huelva.' By James Mason. London. 1858.

'Notes on the Mines of Rio Tinto.' By J. Lee Thomas. London. 1865. 'Memoria sobre las Minas de Rio Tinto:' presentada al Gobierno de S. M. Madrid. 1856.

+ Carte Géologique de l'Espagne et du Portugal.' M. E. de Verneuil et E. Collomb. Paris. 1864.

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