Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/424

 and one foot in diameter. Through the kindness of my learned friend Dr. Bruce of Belfast, a still more perfect specimen from Moira has been deposited in the Ashmolean Museum at Oxford.

The usual forms of these bodies are modifications of those drawn, more or less elongated or compressed. No two of them are to be found exactly alike in all their proportions. Their length commonly varies from one to two feet, their thickness from six to twelve inches. Their substance in all cases is flint. The termination of these siliceous bodies is distinct, and the separation of the flint from its matrix of chalk always clear and decided. Their outer covering has the appearance of a thin epidermis, smooth externally, and whiter than the mass of flint inclosed, which is usually of a dark grey colour. The whiteness of this crust is probably derived from an admixture of lime with the silex, as usually happens in the exterior part of common chalk flints.

In all cases these bodies seem to have had a central aperture passing into and generally through their long diameter. The breadth of this aperture varies in different specimens, from half an inch or less to four or five inches, but is tolerably uniform in each individual. It is usually largest in the elongated varieties; small, and sometimes almost extinct in those of a more compressed form. These cavities are always filled with chalk, of the same nature with the matrix in which the flints are imbedded; they appear to have been filled when the chalk was in a fluid state, and could accommodate itself to all the cavities of the organic body. The upper extremity of this central cavity or pipe is generally terminated by folding itself outwards, so as to form a kind of lip or scroll by its junction with the outer circumference, which is inflected inwards. The lower extremity of the siliceous body is usually contracted,