OF PYRITES AND GYPSUM.
321
In the Coal Measures, the pyrites has no doubt been derived
from circulating ferruginous waters by the reducing action of
the carbonaceous matter. A similar origin may account for the
pyrites found elsewhere in connexion with organic structures, as
in the fossil wood occurring in the cliffs of Walton-on-the-Naze.
Solutions of iron salts, like the carbonate in carbonated water,
brought into relation with sulphates in the presence of decom-
posing organic matter might readily yield pyrites.
Bischof, the great chemical geologist, obtained pyrites by
adding sugar to a solution of sulphate of soda containing a little
iron, and allowing a pitcher of the liquid to remain in a cellar
for four years. A well-known case of the formation of pyrites
was that cited a century ago by Pepys, who having prepared
hydrogen by the action of dilute sulphuric acid on iron filings,
left a jar of the by-product in his laboratory for a year, and then
found that some unfortunate mice which had fallen into the
liquid had effected its reduction to the condition of sulphide, in
the form of pyrites. In nature, ferrous sulphate may occur in
percolating waters and readily suffer reduction by any decaying
organic matter with which it may come in contact. But in such
cases the sulphate itself may have been derived from the
oxidation of pyrites elsewhere ; so that the changes are cyclic.
Pyrites sometimes occurs crystallized in alluvial deposits of
recent age, and is found in the muds forming around the coasts
at the present day. According to the report of the Challenger the
colour of the "blue muds," which are frequently found either in
the deeper waters around continental land or in enclosed seas, is
due to organic matter and to sulphide of iron in a fine state of
division. When first dredged, it emits the odour of sul-
phuretted hydrogen.
Annelids pass mud through their alimentary tract, and during
its passage any sulphates may be reduced to sulphides.
The bluish colour of certain clays like the gault, and some of
the Jurassic limestones like the Forest marble, is usually connected
with the presence of finely-divided iron-pyrites.
The decomposition of pyrites often contributes to the mineral
character of waters percolating through rocks in which it occurs.
It is curious to find that Harrison, the good old rector of Rad-
winter, in Essex, in writing in his quaint history, more than three
hundred years ago, about the waters of Bath, refers their
w