Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/292

 Cork Institution, from whom the specimens that I have seen were obtained, informs me, that it was found at a small distance from the surface, near the base of a hill composed of a flinty slate, and that he has seen it adhering to a piece of rock of that description. But it has occurred principally detached in the form of globular nodules; irregularly grouped together, and of various sizes, the longest about an inch in diameter, externally coated with a yellowish brown earthy crust, and within composed of radiating crystalline spiculæ, the characters of which agree very nearly with those of the wavellite from Devonshire, described by Mr. Davy; indeed some of the specimens from the county of Cork, are scarcely to be distinguished from some of those obtained at that place.

The most distinct specimen that I have seen was a nodule about three-fourths of an inch in diameter, in part affected by decomposition and containing some small spongy cavities. On its external surface indistinct dihedral terminations of the crystalline shoots are discernible; and internally, where it is not decomposed, its lustre is higher and more glossy than is common in the Devonshire fossil. The specific gravity of that part of it, which was very pure and nearly transparent, was 2.34.

The nodules are in some instances decomposed throughout, the spiculæ having lost their lustre, acquire a dull grey or brownish colour, and become much softer than when unchanged ; and Mr. Hincks has seen some of them altogether in the state of clay, apparently from the effect of decomposition.

It would appear that the fluoric acid, of which Mr. Davy has ascertained the presence in the wavellite from Devonshire, exists also in that from Cork; for glass is corroded by heating upon it, in a drop of sulphuric acid, a fragment of the mineral from either of those places.