MODE III. LIMESTONE. 451 



Mr. Kirwan mentions a sky-blue limestone, 

 from Aberthaw, in Glamorganshire. But this, 

 like the blue marble of Narbonne, or the blue tur- 

 quin, appears to be only grey. Dr. Kidd de- 

 scribes it as light blue, or grey ; and says, that it 

 is common in Somersetshire, and that it only 

 occurs in the form of shingle, or large pebbles, on 

 the sea shore at Aberthaw. This colour seems to 

 indicate a mixture of iron ; and such limestones, 

 when calcined, become of a buff colour, and fur- 

 nish a harder mortar than any other. 



STRUCTURE III. CONCHITIC. 



Many of this description belong to the noble 

 division, or marbles, not to mention the konites ; 

 but many also are of a soft and coarse nature, 

 whence they fall under this Structure. The lime- 

 stone containing shells is generally grey, but some- 

 times dull white, or brown. Sometimes even 

 bones are found in limestone, and in marble ; and 

 Faujas, as already stated, has recently observed a 

 remarkable example near Verona, where a fine 

 black marble, containing petrified bones, is worked 

 into large columns. Karsten, as quoted by Gme- 

 lin, has also mentioned a limestone, containing 

 bones, which is found at Erfurt; but the piece* 

 seem to be small and detached. 



2 G2 



