Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/210

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 * 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 cropping out amongst the wood, and resembling a wall about 8 feet high.
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 * 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 cropping out amongst the wood, and resembling a wall about 8 feet high.
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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 frequently 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 laminæ 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 the water impedes their progress. The mine is then deserted, but