chert. Although these patches of compact blue limestone effervesce with acids, they are however hard enough to strike fire with steel. I have seen at the apartments of the Geological Society a series of specimens from the County of Rutland (Nos. 890 to 896), so very much like those of Purbeck and Portland, that they might be taken one for the other. But generally speaking, the texture of the coarse shelly limestone is uneven and rough; it contains a great many shells, which, according to M. Brongniart, belong mostly to the tribe of the littoral shells. Sand also occurs either filling up the cavities of the shells, or dispersed through the substance of the limestone. It is generally calcareous, and of a dirty-yellow colour; sometimes it is siliceous, and then appears under the form of very small brown grains. In the coarse shelly limestone of Swanage, the colour is yellowish grey, and the texture somewhat resembles that of a pisolite; (var. of the oviform limestone of Kirwan). In the quarry of Tilly Wym and in that of Wind Spit, a little westward of the former, it is mostly composed of shells of oysters, which have lost their outside coat. In the Portland stone, judging from the quarry which lies on the north-east of the island, the greatest part of the remains included within it, are casts of a species of Trigonia of Lamarck (Hippocephaloides of Plott) as I am informed by Mr. Parkinson; a genus of which Mr. Péron has found a living species in the Southern Seas. This stone is rather rough, on account of the many cavities left by the casts of the shells, and which cause the air contained within, to oppose a resistance to the hammer, in the manner of the porous lavas: some of those cavities are however lined with crystallized calcareous spar.
