THE ROMANCE OF COAL AND OIL 
They are called index fossils. With their help it is pos¬ 
sible to compare horizons in distant localities, and often 
to determine to which geologic age the section belongs. 
It is true that the fossil shells and other small marine 
animals that are embedded in limestone and sandstone 
supply many more index fossils than do the plants of 
the shale and the sandstone. The lower animals, or in¬ 
vertebrates, will therefore always be more important for 
historical geology than the plants, but in a number of 
instances, especially in many horizons of the Pennsyl¬ 
vanian and Tertiary periods, plants are the only index 
fossils we know. 
Sometimes it is extremely important to know to just 
which geologic age certain rocks belong. For instance, 
some time ago a petroleum company was drilling for 
oil in Louisiana. The drillers had gone down over four 
thousand feet, and drilling had become very expensive. 
The oil-bearing sands in that particular locality are 
found in the Lower Cretaceous formations. Immediately 
below the Lower Cretaceous rocks are the Pennsylvanian, 
because there the intermediate formations are missing. 
Such a break in the series is called by geologists an un¬ 
conformity. It was of utmost importance for the oil 
company to know whether they were still drilling in 
the Lower Cretaceous rocks or whether they had already 
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