ORIGIN OF PETROLEUM. 
61 
ORIGIN OP PETROLEUM. 
What is the origin of the gas and the oil? We 
maj take a piece of soft coal and heat it in a retort. 
We obtain gas, oil, and a black tarry substance called 
bitumen, besides a great many other products. It is 
often supposed that petroleum and natural gas have 
been produced in a similar way from the coal. The 
heat in the earth is thought to drive off the gas and 
the oil from the soft coal, leaving it anthracite. But 
this does not seem to be the case ; oil is not always 
found near coal-seams, nor do oil-bearing rocks always 
give evidence of great heat. It is quite probable that 
gas and petroleum were produced independently of 
the coal; and since salt beds are usually found close to 
the oil, the theory is that it is formed at ordinary tem¬ 
peratures by organic matter. The coal was formed 
from land plants in the presence of fresh water; the 
oil from organic material in the sea in the presence of 
salt water. 
It appears that marsh gas, CII4, is constantly es¬ 
caping from the petroleum. In this way the bitu¬ 
men in the neighborhood of salt beds was probably 
formed. Professor Liebig suggests that the diamond 
itself is formed by crystallization from petroleum ot 
greater or less density. 
During the Carboniferous age the land areas were 
not much increased. There was comparatively little 
land. The mountains were low, so that there were no 
snow-capped peaks to chill the atmosphere and cause 
winds to any great extent. The climate, therefore, 
