CHAP. XV.] 



EXPLOSION OF GAS. 



215 



Vegetable Structure of Mineral Charcoal from Clover-hill Mines, Virginia. 



coal of the Blackheath mines, and the annual quantity taken by 

 Philadelphia alone, has of late years amounted to 10,000 tons. 

 We miufht have expected, therefore, that there would be danger 

 of the disengagement of inflammable gases from coal containing 

 so much volatile matter. Accordingly, here, as in the English 

 coal-pits, fatal explosions have sometimes occurred. One of these 

 happened at Blackheath, in 1839, by which forty-five negroes 

 and two white overseers lost their lives ; and another almost as 

 serious, so lately as the year 1844. 



Before I examined this region, I was told that a strange 

 anomaly occurred in it, for there were beds of coke overlying 

 others consisting of bituminous coal. I found, on visiting the 

 various localities of this natural coke, that it was caused by the 

 vicinity or contact of volcanic rocks (greenstone and basalt), 

 which, coming up through the granite, intersect the coal- 

 measures, or sometimes make their way laterally between two 

 strata, appearing as a conformable mass. As in the Durham 

 coal-field in England (in the Has well collieries, for example), the 

 igneous rock has driven out all the gaseous matter, and, where 



