of Hampshire and Dorsetshire, 257 



correctly, with the slate-clay, or shale (S chief er-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 lime- 

 stone 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 J* and of this latter to a beautiful transparent calcedony. 



* Though the worth Jlint 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 

 $ome 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 cliert is not generally 

 to be found in distinct globular masses as flint is, but rather in continuous layers, separat- 

 ing thicker strata of rocks. 



2 K 



