1868. | On the Iron-pyrites Mines of Andalucia. 471 
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 doubt- 
less 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 con- 
figuration 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 pre- 
cautions 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. Hntering 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 
