THE ROMANCE OF COAL AND OIL 
some irregularity of the rocks, there is accumulated, un¬ 
der enormous pressure, an oil pool. It is usually com¬ 
bined with gas and also with salt water. When a drill 
penetrates to this oil accumulation the enormous pres¬ 
sure forces out the oil or gas or the salt water, and we 
have a “gusher.” Sometimes the rocks are folded to 
form a dome which looks more or less like a cup upside 
down, and then there is a splendid chance for the accu¬ 
mulation of oil under the top of the dome, provided 
that mother rocks are sufficiently near to supply the oil. 
There are some great beds in California composed 
of diatomes, which are microscopically small unicellular 
algae, contained in silica capsules. These beds are a 
mass of white powder several miles long and several miles 
thick, lying below the surface of the earth. The oil 
which the living diatomes contained has passed into 
some of the California oil sands which are near by. Deep- 
sea algal accumulations have also produced some of our 
deepest and richest oil pools, formed as the ocean bed 
was filled up with limestone, sandstone, and shale masses. 
These are the sources of the chief vegetable oil pools which 
remain for the world’s use. 
The use of mineral oil has been enormously increased 
during the last decade. The world production up to 
and including the year 1925 amounted to 13,500,000,000 
123 
