Carbonate of Iron in Rhomboidal spiculæ
This brown Carbonate of Iron encrusts the columns inside the Highgate and Sheppey Septaria; at the latter place it is called waxen vein, see tab. 172: a term the appearance of some varieties seems to warrant. It is not long since most of the linings of Septaria have been proved to be Carbonate of Iron with Carbonate of Lime sometimes filling the interstices; some of those of Suffolk and a few other places, have Carbonate of Lime only. Such as tire found in Derbyshire, above the Coal measures or Coal formation, called Iron stones, (see tab. 61.) have whitish Carbonate of Iron in the form of Pearly spar in the divisions, which is generally understood. Carbonate of Iron in its present state, however, is not so generally known as not to require figuring here. The colour varies much as well as the form, the surface being sometimes smooth; at other times beautifully crystallized in spicules of various dimensions; some so minute as to give a plush or fine velvety lustre, which is easily disturbed by handling, particularly when fresh gathered. The present specimens are from High gate Tunnel. Some specimens had upon them the stellæ of Sulphate of Barytes, called Lepastri, see tab. 172, but they were rare at High gate. It may be worthy of notice that many of the cavities of the Septaria lined with Carbonate of Iron, contained a quantity of water apparently very pure, some of which 1 have had four years in a phial, gathered by B. G. Snow, Esq. who paid so much attention to that place; it should seem that the ingress and egress of the water formed the deposit or lining of Carbonate of Iron, pyrites, &c. and I presume much depended upon the seasons whether this water came loaded or not with substances to deposit, as these linings seem to have been deposited at various times and are often various in the same stone. The upper figure is from a velvety specimen; the lower one shows the spiculæ arranged in tubercles; this kind is sufficiently hard to bear brushing without injury.