418 ELEMENTARY GEOLOGY 
were now developed in greater variety. Flying insects 
appeared (chiefly Orthoptera and Neuroptera), and some 
of them were of great size, having a spread of wings of 
one or two feet. Among the Devonian insects there 
was a notable absence of those interesting forms of 
bees, ants, butterflies, etc., which are now so abundant. 
These creatures, many of which depend upon plants 
and flowers for their existence, could not develop until 
vegetation had attained a sufficiently high character for 
their existence. The evolution of plants and insects 
has been continued side by side. 
In the rocks of this and other ages, there are deposits of 
natural gas and oil, or petroleum, which owe their origin to the 
existence of animal and plant life upon the globe during past 
ages. The remains of these organisms accumulated among the 
rocks that were deposited in the seas or lakes, and in the course 
of ages, by slow distillation, the carbon and hydrogen of their 
bodies have in part been accumulated in the form of hydro- 
carbons, either of the gaseous or liquid kind. Wherever this 
decay of organic life is in progress, hydrocarbons are produced. 
This is well illustrated in the marsh gas which rises from the 
decay of vegetation in swampy places. Commonly the decay 
product escapes into the air, and hence is lost to man. But under 
some conditions, the hydrocarbons are allowed to accumulate in 
basins, some of which have since been pierced by wells. 
The most favorable condition is a reservoir of porous rock 
capped by an impervious layer. The gases and liquids, derived 
from the distillation of the organic remains, rise into the porous 
rock and are prevented from escaping by the dense layer above. 
When this natural reservoir is pierced by a well, the pressure of 
the rock and gas, forces the substance to the surface, 
