Sulphuret of Iron; Iron Pyrites

Octaedral Pyrites is not so common as cubical Pyrites; we have it however along with various substances, as calcareous spar, limestone, coal, &c. The present figures are designed to show this modification from the cube passing into what Haüy calls the cubo-octaëdre, thence into the perfect octaëdron. At the commencement of this change the corners of the cube are replaced by triangular faces—see the left hand figure—which, as the modification goes on, becomes planes of six sides each—see the middle figure—and at last the primitive faces are lost. These six-sided planes are reduced again to triangular ones, forming the octaëdron.

I am indebted to the late Lady Elizabeth Noel of Bath for the upper and right hand specimens. The first is the cast of a shell of the Trochus genus; and it should  seem that the crystals are on the cast in place of the shell, as the rock is about the thickness of the shell from the cast, and is a mould of the outside of the shell. On the left hand side remains a bit of the rock, and on the right hand are exhibited the thickness and calcareous remains of the shells, sufficient to indicate the species to a conchologist, which appears to be different from any shells of the present age. This is taken from another specimen which also came from bath by favour of T. Walford, Esq. The pyrites on this are octaëdrons, some of which have their solid angles slightly truncated. It nearest resembles Trochous niloticus Linn., but we do not consider it as that species.

The cast of the shell on the right hand, of a golden hue, being covered with pyrites, generally deeply truncated, seems a species of Mytilus cut off in the manner of Donax denticulata Linn. The shell on the left hand seems to be a Tellina, and includes pyrites, chiefly of a cubo-octaëdral form—see left hand bottom figure—as it were hermetically sealed in, and of course not discovered till the shell was broken. How these crystals came there may excite wonder; but were we sufficiently acquainted with nature’s operations, we should see every natural cause as well as its effect. This shell most nearly resembles Lister’s Tellina lata rugosa, tab. 390. f. 229.

These fossils are in great abundance above the sand quarries at Woolwich and Charlton, about nine feet from the surface of the hill, in a loose marly stratum, from one to six feet thick.

How long they have been preserved there is not known; they however will soon rot and decay after exposure to the air. The other sorts of shells are two species of Turbo, probably of the same date; these will also fall to pieces. Of oyster shells there is great abundance, which do not, to my knowledge, differ from those at present known, nor do they decay so readily as the others. There are other shells in this curious place, and in Lady Wilson’s park at Charlton, with specimens of which I have been favoured by her ladyship. I have gathered the more common ones myself.