The method now adopted of working the lenticular deposits of pyrites occurring in the province of Huelva by open cutting and quarrying, is favourable to the examination of these singular masses of mineral. The latter run parallel with the strike of the beds; they are generally, but not always, in close proximity to greenstone, the greenstone often forming one wall or sahlband; they usually, but not always, decrease in width as they go down, and sometimes end in a boat form; they are generally, but not always, marked by oxidation on the surface, and generally, but not always, by a depression between the two walls. The latter phenomena, have been supposed to have been occasioned by the ancient mining for copper having worked away the upper surfaces and exposed fragments of mineral to oxidation; but the facts appear to be at variance with the causes thus assigned, inasmuch as there is extensive colouration where there have been no workings; and on the other hand, no subsidence in many cases where ancient workings exist.