V 



Sect VIIL] 



MINERALOGY, 



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have been subsequently removed, upon an extension of 

 the same beds we may often see good workable codL 

 Thouo-h in cliffs, either on the shore or on the sides of 



often 



rivers, hills, and mountains, we commonly find the 

 direct evidence of the existence of coal, it may be 

 traced to its beds, where such occur, by means of the 

 detritus brought down by brooks and rivers. By follow- 

 ing rolled pebbles up such water-courses they may be 

 often seen to end near some bed or beds whence they 

 have been derived. If these cross the stream, a good 

 opportunity may be afforded for es:amining their quality 

 and thickness. The pebbles may, however, come from 

 the sides of some adjacent hills sloping towards the 



streams, the beds of coal not crossing them, fragments 



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only of their outcrop being mingled with any others of 

 associated beds. The thickness of such coal-beds may 

 be thus concealed, as will be readily seen by the annexed 

 section, in which a represents the river course, up which 

 pebbles of coal may be traced ; b b beds of coal, the out- 

 crops of which, c c, may be much concealed by fragments 



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of rock descended from above and mingled in a frag- 

 mentary covering d d. The best should be done to 



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obtain a knowledge of the associated beds by tracing up 



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