correctly, with the slate-clay, or shale (Schiefer-thon of Werner) with which it alternates.
IV. Calcareous Sandstone.
This is coarse-grained, loosely aggregated, of a yellowish-white colour, with small brown grains of siliceous sand, and some specks of mica; it effervesces briskly with acids; it is disposed in strata of several yards in thickness which alternate with coarse shelly limestone and thin layers of chert; it enters into the composition of the cliffs of the south-western and southern coast of the Isle of Wight. As it is easily acted upon by external agents, it is often hollowed out, leaving the interposed strata of coarse shelly limestone, and the layers of chert, like shelves or overhanging cornices projecting from the cliffs, till, the sandstone giving way to a great extent, the upper strata fall down, and take in settling all degrees of inclination. This is the only way to account for those large and numerous blocks which have encumbered the under cliff in the Isle of Wight. I have observed in many instances the passage of the sandstone to chert,[1] and of this latter to a beautiful transparent chalcedony.
- ↑ Though the words flint and chert are pretty often used indiscriminately, I do not by any means consider them as synonymous: chert, I believe, is a kind of hornstone, the fracture of which is between scaly and flat conchoidal: it has a somewhat drier aspect, and is more generally of a greyish colour, or variegated white and brown: such are some of the characters clearly made out by Mr. Kirwan. Elements of Miner. vol. i. p. 303. There is besides, I think, a geological character, viz. that chert is not generally to be found in distinct globular masses as flint is, but rather in continuous layers, separating thicker strata of rocks.
