THE SOURCES AND USES OF IRON PYRITES. 
115 
and oxidation of its associated iron. The Buitron Company, 
whose mines are situated in the same province of Huelva, also 
for a short time contributed a considerable amount of pyrites to 
this market, but the largest undertaking in connection with 
mining for this mineral is unquestionably that of the Rio Tinto 
Company, commenced in 1873. 
The deposits belonging to this association are situated near 
a town of that name, 84 kilometres, or 52 miles, north of the 
port of Huelva, at the mouth of the river Odiel, with which they 
are connected by a railway. In this part of Spain the 
deposits of cupreous pyrites consist of a series of more or less 
continuous lenticular masses of ore, possessing a general direc- 
tion a little north of east, and south of west, extending from 
Aznalcollar near Seville, in the east, for a distance of more than 
70 miles westward, to the well-known mines of San Domingos, 
within the Portuguese frontier. 
The substance of these deposits consists of an intimate and 
compact admixture of iron pyrites with a little copper pyrites 
through which strings of the latter mineral are sometimes ob- 
served to ramify ; small veins of black sulphide of copper less 
frequently traverse the mass. Gralena and blende are also 
present, as well as arsenical pyrites in small quantity ; and in 
the ancient workings great quantities of blue and green vitriols 
are met with, as a product of oxidation of the ore by the action 
of air and water. 
As already stated, these lodes may be regarded as enormous 
lenticular masses of cupreous pyrites* in which small strings of 
copper ores containing from 5 up to 40 per cent, of that metal 
are occasionally met with, but more particularly in the vicinity 
of the walls and in the neighbourhood of quartz veins. This 
fact was evidently known to the Romans, who entirely confined 
their operations to such rich ores, and neglected the general 
mass of the lodes which does not contain much above 2 per 
cent, of copper. 
The prevailing rocks throughout the entire district are clay 
slates, which, from some imperfect fossils recently discovered 
in them, would appear to be of Silurian age. These slates are 
in places disturbed by dykes of diabase and quartz-porphyry, 
the latter rock frequently forming one of the walls of the metal- 
liferous lode. 
These deposits have, almost without exception, been wrought 
in remote antiquity, the workings having in some exceptional 
cases reached a depth of nearly 50 fathoms beneath the surface. 
The larger proportion of these ancient excavations are mani- 
festly of Roman origin, although the occasional discovery of 
stone implements would appear to indicate that the ferruginous 
capping of the veins furnished a red pigment to the early in- 
