Calcareous Schist

The term Slate is applied to very different Stones; but it is more generally understood as belonging to the blue or gray Slate, commonly so called when it is even a term for colour, as Slate-colour; but this term does not lead to accuracy, as Slate is found to vary in this respect. Colour formerly helped to distinguish the species for common use; but now many varieties, both in colour and texture, are used! While Slates are common in some counties, in others the people are strangers to them j and luckily find Limestone so fissile as to serve for roofing their houses; and this, being schistose or slaty, has obtained the appellation of Slate.

The tower figure came from Stoncsfield, Oxfordshire, and has often in its fissile divisions shells, sharks’ teeth, and other things of this nature. These Stones often have fishes’ teeth in them, perhaps most remarkable at Verona in Italy. When Stones for building are chosen from these quarries, it becomes necessary to place them nearly as they were in their native places, else by exposure they split perpendicularly, and fall from their places in flakes; which may occasionally be seen in Limestone buildings.

The small upper specimen was found about 2 or 3 feet from the surface in Sussex, and furnishes strong argument for its very recent formation. Little petrified shells, resembling the minute stagnant pool muscle or bivalve, if not one of the same species as now exists, cover it in every division very abundantly; and like other fissile Stones, the bivalves show the convex on one side and the concave on the other. I am obliged to my friends Messrs Borrer and Weeks for the specimens in my cabinet. The next specimen is from Stoncsfield, a quarry famous for a great variety of animal exuviæ, especially of the genus Cornu ammonis. This has an alligator’s tooth finely preserved. If we may be allowed to judge from the teeth, there have been many species of alligators enveloped in the catastrophe that made such wonderful havock. The other two pieces, the upper of which is without any appearance of petrifaction, are a sort which is often divided by very silvery-looking Mica covering the fissile surfaces, and mostly accompanying the more sandy sort:—we find by Lady Wilson that it approaches even to a Jasper. The lower specimen has casts of different bivalves, and has a hole in the top, which shows in a small degree the manner of manufacturing them for Slates. Several of the above specimens contain a considerable portion of Sand. All Limestone quarries have a more fissile Stone near the top: those of Portland, Purbeck, and Bath are generally shelly, and sometimes merely a congeries of compressed shells almost as thin as paper.