' GEOLOGY. 259 



It partially dissolves in the lighter hydrocarbons from coal and 

 petroleum. In petroleum naphtha, of 39.67 per cent, of dark 

 brown bitumen separated from residuary humus, one hundred 

 parts afforded when distilled — 



Moisture 0.33 



Bitumens and Gas 77.67 



Carbon as Coke 20.80 



Ash 1.20 



100.00 



— Proc. of Boston Society of Natural History, 1866. 



FORMATION OF COAL FROM PETROLEUM. 



One of the more genei'ally accepted theories respecting the 

 foi'mation of petroleum, supposes that substance to be a pro- 

 duct of the destructive distillation of coal by means of the earth's 

 internal heat. There is being discussed just now, however, a 

 theory which is the exact converse of this, — a theory, according to 

 which, instead of petroleum being formed from coal, coal was 

 formed from petroleum. It is well known that " all organic sub- 

 stances which ai'e not themselves volatile, such as wood, desh, and 

 other vegetable and animal matters, yield, when subjected to tlie 

 influence of heat below dull redness, tarry oils, having in all cases 

 the general character of petroleum, and differing only according 

 to the specific differences in the materials from which they may 

 have been obtained ; " and the new hypothesis supposes that the 

 materials from which our coal-beds were formed were converted 

 in the first instance into such " tany oils," and that these oils, 

 under the long-continued action of heat, gradually lost nearly all 

 their oxygen and the chief part of their hydrogen, the residuum 

 gradually becoming solid. The advocates of this theory point in 

 sujoport of it to the phenomena presented by the celebrated " Pitch 

 Lake" of Trinidad. This lake covers an area of ninety-nine 

 square miles, and is of very great depth. " The bitumen is solid 

 and cold near the shores of the lake, and gradually increases in 

 temperature and softness towards the centre, where it is boiling. 

 The ascent to the lake from the sea, a distance of three-quarters 

 of a mile, is covered with a hardened pitch, on which trees and 

 vegetables flourish ; and aljout Point la Braye the masses of pitch 

 look like black rocks among the foliage." Mr. G. P. Wall 

 describes the lake as yielding three kinds of asphaltum : "1. As- 

 phaltum glance, which is hard and brittle, of an intensely black, 

 brilliant lustre, and conchoidal fracture. 2. Ordinary asphaltum, 

 of a brownish-black color, containing 20 to 35 per cent, of earthy 

 admixture and a considerable proportion of water, and jjossess- 

 ing the property of plasticity, which it gradually loses on long 

 exposure to the sun and atmosjihere. 3. Asphaltic oil, occurring 

 associated and diluted with water, but appearing, when concen- 

 trated, as a dense black fluid with a powerful bituminous odor. 

 If collected in an open vessel, the more volatile part of this oil 

 evaporates after a few months, leaving a solid black substance. 



