200 Dr. Bright on the Strata 



16. A thin bed of limestone breccia containing rounded 

 pebbles, and organized substances resembling the palates of 

 fish. This bed has not been traced on the southern bank. 



17. Limestone highly crystallized, containing much iron, 

 and composed in a great measure of organic matter. It is 

 seen very distinctly on the northern bank of the river crop- 

 ping out amongst the wood, and resembling a wall about 

 8 feet high. 



The limestone of St. Vincent's rocks when calcined yields a very 

 pure lime : large quantities of it are exported for the use of the 

 sugar works in the West Indies, in an unslaked state and packed in 

 tight casks, and it is used extensively for building. All the roads 

 in the neighbourhood of Bristol are repaired with the limestone, as 

 are those in the neighbourhood of Gloucester, whither it is conveyed 

 by means of the Severn. 



Calamine is the most important mineral In point of frequency and 

 value that the limestone yields. It is found in veins of calcareous 

 spar crossing the limestone, accompanied by heavy spar and fre- 

 quently by galena. The calamine is either amorphous or assumes 

 the form of calcareous crystals which have been encased by it and 

 since removed. The calcareous crystals in these veins have generally 

 the dog's-tooth form ; the heavy spar is not crystallized, but appears 

 fibrous or composed of thin laminse lying above one another. The 

 galena sometimes presents very regular cubes, the angles of which 

 are generally truncated. The calamine has hitherto been worked in 

 a very imperfect manner: the vein is broken into, when it meets 

 the surface ; a rough windlass is placed over the hole, and a 

 bucket is attached to a few fathoms of rope ; two or three men 

 work at the vein as long as the ore is found in abundance, or until 

 thq water impedes their progress. The mine is then deserted, but 



