OF PYRITES AND GYPSUM. 319
The tendency to limonitization, though common enough in
some kinds of pyrite, is even more marked in marcasite. Thus,
the well-known nodules from the chalk, often called "thunder-
bolts" and "fairy balls," are usually converted superficially into
limonite, so that the appearance of the globular masses is rather
suggestive of rusty cannon-balls. In some cases the ochreous
oxide forms only a thin brown rind, whilst in others it may
occupy the greater part, or even the whole, of the nodule. If
there is any fissure, the meteoric agents may gain access to the
interior and attack the mineral in the immediate neighbourhood
of the crack. The nodules tend to break readily in the direction
of the radiating fibres, and these fibres lose their brass-like
appearance when atmospheric influences penetrate along the
planes of weakness.
Not only may pyrites suffer hepatic alteration, or limonitization,
but it may also undergo a saline decomposition, sometimes called
vitriolization. This type of change is sometimes very con-
spicuous in marcasite, and, as every one knows, is a source of
great inconvenience to the collector of pyritized fossils. The
iron sulphide in the presence of moist air becomes oxidised-'
to ferrous sulphate, which is the salt known, when formed
naturally, as melanterite, and when formed artificially as "green
copperas" or "green vitriol." There is also frequently formed,
concurrently with the melanterite, free sulphuric acid ; and to
this product may be ascribed the corrosive action too often seen
in our cabinets and museums. The labels, the cardboard trays,
even the wooden tablets and the drawers themselves, may be
attacked more or less seriously by the action of this acid
eliminated by the decomposition of the pyritous fossils.
Although the name melanterite suggests a black mineral, the
ferrous sulphate is normally green. Being a very unstable body,
it tends to pass readily into the state of ferric sulphate, or per-
sulphate of iron, a yellowish substance often seen as an
incrustation or effloresceuee on rocks containing decomposing
pyrites. Moreover the ferrous sulphate also passes by oxidation
into the condition of ferric hydrate, as seen in common copperas
on exposure to air. 
It should be noted that Mr. H. N. Stokes shows that the old
notion that marcasite tends to saline change and pyrite to
limonitization is quite untenable. He has observed rapid