58 
MAURICE G. MEHL 
brian and igneous rocks may at once be designated as impossible 
territory. The proportion of the possible productive territory 
to the total area in each zone differs greatly. Again the differ- 
ences favor the northern zone for the proportion of the Post- 
Cambrian area to the total land surface is much greater in this 
belt (see plate XIII). Furthermore, the ^^possible’^ territory in 
the other zones includes considerable areas that may properly 
be eliminated because of the presence of sediments representing 
periods that are generally barren throughout the world, such as 
the broad expanse of supposed Triassic rocks in northeastern 
South America and the great areas of Mesozoics in eastern Aus- 
tralia and central Africa. 
The foregoing considerations are all, in a sense, passive agents, 
factors which would tend to minimize the importance of petro- 
leum accumulations outside of the northern belt. So closely 
confined to this belt are the great accumulations as they are 
known today that it is thought there must be a more active 
determining factor; one of the fundamental influences in the 
formation of petroleum. Of these, one of the most far reaching 
is probably the temperature consideration. 
It may safely be granted, perhaps, that petroleum is derived 
from organic matter. Without going into the evidence it may 
also be stated that there is reason for assuming that no par- 
ticular group or groups of organisms may be designated as the 
source of petroleum except that all evidence points to the fact 
that it is only the smaller plants and animals, perhaps largely 
the microscopic forms, and fragmental material from larger 
organisms that are available. 
It is evident that there is a marked dependence of life on 
temperature conditions. Perhaps this one factor, more than 
any other, determines the variety and abundance of life through- 
out the world. It is recognized that the amount of agitation of 
the waters, the degree of salinity, the nature and amount of 
sediment, the depth of the water, etc., are very important factors 
in the distribution of marine life, but for every temperature 
these other conditions may be found in almost endless variety 
and combination. Still, life is not equally abundant in every 
