706 



THE WONDERS OF GEOLOGY. Lect. VII. 



of decay. Diamonds cannot have been produced by the 

 action of fire ; for a high temperature and the presence of 

 oxygen gas, would call into play their combustibility. But 

 there is the greatest reason to believe that they have been 

 formed in the humid way — that is, in a liquid ; and the 

 process of decay is the only cause to which their formation 

 can with probability be ascribed."* 



The matrix of the diamonds of Southern India is the 

 sandstone breccia of the clay-slate formation. Captain 

 Franklin discovered in Bundel Kund, diamonds imbedded 

 in sandstone, which he supposed to be the same as the New 

 Red-sandstone, for there were at least 400 feet of that rock 

 below the lowest diamond beds, and strong indications of 

 coal underlying the whole mass.f 



29. Anthracite, Plumbago, &c. — The coal commonly 

 used for domestic purposes in this country is the bitumi- 

 nous, containing, as previously stated, a volatile inflam- 

 mable fluid in its cellular structure. The Anthracite^ 

 culm, or stone-coal, is coal deprived of its bitumen, by the 

 causes already explained. When coal is in contact with 

 trap, or basalt, it is often in the state of anthracite ; 

 while the layers in immediate contact with the volcanic 

 rocks are charred, and in some instances, the mass is 

 converted into plumbago, or graphite ; the substance used 

 for drawing-pencils. By a series of interesting experi- 

 ments, that eminent chemist and geologist, Dr. Macculloch, 

 demonstrated the transitional changes from bitumen to 

 plumbago. Hydrogen predominates in the fluid bitumen ; 

 bitumen and carbon in coal ; in anthracite bitumen is alto- 

 gether wanting ; and in plumbago, the hydrogen has also 

 disappeared, and carbon only or chiefly remains. 



In America, from the prevalence of anthracite in the 

 carboniferous deposits, this substance is in universal use ; 



* Liebig's Chemistry, vol. i. p. 336. 



f London and Edinburgh Journal, October, 1835. 



X The name is derived from a Greek word, signifying coal. 



