318
ON THE NATURAL HISTORY
are sometimes known from their liver-brown colour as Hepatic
Pyrites.
In mineral-veins, where pyrites is often abundant, the
production of iron hydrate gives a rusty appearance to the vein
stone, and in the case of quartz reefs it suggests a kind of
calcination leading to the term "baked quartz." By removal of
the pyrites, cubic cavities are left, containing more or less
ochreous iron-ore. Where the pyrites is auriferous, the precious
metal by resisting meteoric influences may remain in the form of
sprigs or plates or grains, and hence it comes about that native
gold is frequently found in the rusty quartz. Professor H.
Louis has called attention to the fact that the iron sulphide in
gold-reefs is invariably cubic pyrite—never marcasite.11
The "Essex gold-mine," often referred to by old writers, was
no doubt mythical. Yet it is not unlikely that some of the pyrites
of this county may contain a trace of the noble metal. We have
already seen that Dale refers to its existence. He may have
been right ; yet there is no reason to suppose that the pyrrite
minerals of the south-east of England contain more than a
minute proportion of gold, if any. Old Sir John Pettus in his
"Essays on Metallick Words," published in his Fleta Minor, in
1683, says, "they are excellent fire-stones which we find in our
mines in England, but not so good for fire-locks as those which are
brought from Germany, etc. And our Marcasites do neither
afford gold nor silver worth the charge."12
In mineral veins the alteration of the pyrites into limonite
occurs only in the upper part of the lode, above the water-level
of the country, or in what P6sepny has called the Vadose
region. Below the plane of saturation, the metallic sulphides
may retain their original character. The decomposition of the
pyrites on the "back," or outcrop of the vein, gives rise to that
ferruginous mass which the Cornish miner calls "Gozzan," and
the Continental miner the Chapeaude Fer or the Eiserner Hut.
Probably in many cases ferrous carbonate is formed as an
intermediate product in the production of ferric hydrate.
11 "On the mode of occurrence of gold." Mineralogical Magazine, vol. x. (1893),
p. 241.
12 The title of this book Fleta has reference to the Fleet Prison, in which the work was
written by. Pettus, whilst confined, as he put it, "for my being too kind to others, and too
unjust to myself." As there was a legal work entitled Fleta, also because written in the
prison, the term minor was added by Pettus partly for sake of distinction and partly as a
play on the word miner. Pettus's puns were exceptionally weak.