470 On the Iron-pyrites Mines of Andalucia. [ Oct., 
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. 
Fic. 1. GENERAL Secrion at Rio Tryto.* 
1, Porphyry. 2. Clay-slate, &. 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 copper- 
pyrites, 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 homo- 
geneous 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 rang- 
ing 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 “ Sahl- 
band,” 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 
“ oossan ” 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 thecentre.* 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. 
