FERNS, FOSSILS AND FUEL 
tion of coal. We shall probably not be much mistaken 
if we assume that in almost all cases autochthonous for¬ 
mation took place, although, there are few, indeed a very 
few, minor coal beds where an accumulation of plant mat¬ 
ter carried into a water basin is obvious. 
Some petroleum pools probably are due to an accu¬ 
mulation of vegetable oils, but the majority originated 
from the fats of marine animals. Formerly it was gener¬ 
ally assumed that all oil came from animals, but recent 
microscopic examination and treatment under pressure 
and high temperature of the so-called mother rocks of 
oil prove that some of them are full of algal bodies. 
A mother rock is a rock in which the oil originated. 
Usually it is oil shale. High pressure has driven the 
oil out of these rocks into the porous sandstones which 
frequently accompany the mother rocks. The oil moves, 
driven by rock pressure and water pressure, into the 
sandstones, which are called oil sands. The oil currents 
move through these oil sands, and their discovery is 
the great game of the oil companies, whose geologists’ 
job is to find the oil pools, sometimes at depths of seven 
thousand feet. Whenever an oil current in its circula¬ 
tion has been stopped by an impenetrable rock cover, or 
wherever it has moved more freely in a crevice filled 
with very porous material until it is stopped again by 
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