GEOLOGY. 



89 



an overflow in 1812, amounted to 8,000 cubic feet per minute. 

 The raft thus collected at the mouth of the Red River, is sixty 

 miles long*, and in some parts fifteen miles wide." The quan- 

 tity which descends the Mississippi in a few years might 

 furnish sufficient matter for the largest coal bed known. 



The varieties of coal are brown coal, or lignite, bituminous 

 coal, anthracite coal, and graphite, or black lead : this consists 

 of carbon and iron, and, according to Dr. Hitchcock, "appears 

 to be anthracite which has undergone a still further minerali- 

 zation." All these varieties of coal occur in seams or beds, 

 interstratified by sandstone and shales: brown coal is found 

 mostly in the tertiary, bituminous in the secondary series, and 

 also with new red sandstone and clay. 



Anthracite is found in graywacke, mica slate, limestone, 

 gneiss, plastic clay, and almost all stratified rocks. 



[Fig. 7 is a sketch of the great coal basin of South Wales, in Great 

 Britain, which contains seventy-three beds of coal, whose united 

 thickness is ninety-three feet.] Hitchcock. 



*8 



