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chlorite schist. Grauwacke, when it contains 
few fragments, and exists in large masses, is often 
a metalliferous rock. The precious metals, like- 
wise iron, lead, and antimony, are found in it : 
and sometimes it contains veins, or masses of stone 
coal , or coal free from bitumen. Limestone is the 
great metalliferous rock of the secondary family ; 
and lead and copper are the metals most usually 
found in it. No metallic veins have ever been 
found in shale, chalk, or calcareous sandstone ; 
and they are very rare in basalt and siliceous sand- 
stone.* 
In cases where veins in rocks are exposed to the 
atmosphere, indications of the metals they contain 
may be often gained from their superficial appear- 
ance. Whenever fluor spar is found in a vein, 
there is always strong reason to suspect that it is 
associated with metallic substances. A brown 
powder at the surface of a vein always indicates 
iron, and often tin $ a pale yellow powder lead ; 
and a green colour in a vein denotes the presence 
of copper. 
It may not be improper to give a general descrip- 
tion of the geological constitution of Great Bri- 
tain and Ireland. Granite forms the great ridge 
of hills extending from Land’s End through 
Dartmore into Devonshire. The highest rocky 
strata in Somersetshire are grauwacke and lime- 
stone. The Malvern hills are composed of granite, 
sienite, and porphyry. The highest mountains in 
* Fig. 16. will give a general idea of the appearance and 
arrangement of rocks and veins. 
N 8 
