of Hampshire and Dorsetshire, 259 



chert. Although these patches of compact bkie 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 speci- 

 mens from the County of Rutland (Nos. 890 to 896), so very much 

 like those of Purbcck 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 oi 2, 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 (Hippocepha- 

 loides of Plott) as I am informed by Mr. Parkinson ; a genus of which 

 Mr. Peron 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 op- 

 pose a resistance to the hammer, in the manner of the porous lavas : 

 some of those cavities are however lined with crystallized calcareous 

 spar. 



2 K 2 



