320
ON THE NATURAL HISTORY
vitriolization in pure pyrite. Moreover he finds the fibrous
structure, contrary to usual belief, much more common in pyrite
than in marcasite.13
During the oxidation of pyrites heat is evolved, and cases
have occurred in which bituminous shales rich in pyrites have
caught fire spontaneously from this cause. A famous case
occurred in the cliffs of Kimeridge clay at Ringstead Bay,
opposite Weymouth, in the autumn of 1826. The alum-shale of
Whitby is a bituminous pyritic rock in the Upper Lias, which
owed its economic value to the ready decomposition of its
pyrites.
Iron-pyrites is a common mineral in beds of coal, and the
oxidation of the pyrites when the coal is stored in the bunks of
ships has often given rise to so elevated a temperature as to con-
tribute to the spontaneous combustion of the coal, especially in
hot climates, as when vessels are passing through the Red Sea.
The actual cause of ignition, however, is probably to be sought
elsewhere.
The pyrites is often seen in brassy films on the joint-faces
of pieces of coal in the domestic scuttle. Darwin, in his "Botanic
Garden," has a poetic reference to this pyrites :
" Hence sable coal his massy couch extends
And stars of gold the sparkling Pyrite blends."
The mention of "stars of gold" reminds us that the
inexperienced prospector has occasionally mistaken pyrites for
the precious metal ; and hence in mining camps, where speech
is apt to be more direct than courteous, the mineral has been
called "Fool's Gold."
In coal mines the nodules of pyrites are often known as
"brasses," though a so-called "brassy coal" in some cases
contains very little pyrites. In certain collieries the pyritic
lumps are picked out, and sold for sulphuric acid manufacture.
The coal brass is usually marcasite and not true pyrite, though
cubic crystals are occasionally found.
It is worth noting that pyrites occurs in coal not only in
brass-like lumps and bands, but also in a finely-divided form, so
intimately associated with the coal that it fails to appeal directly
to the eye ; in this form it has been called "black pyrites."14
13 Bulletin of U.S. Geol. Survey, No. 186, 1901.
14 Royal Commission on Arsenical Poisoning, Blue-book of Evidence, vol. ii., 1903.