GEOGEAPHIC DISTRIBUTION OF PETROLEUM 
57 
more than imaginary lines of geographic reference, each does, 
in much probability, mark the average position of some isotherm 
as it has shifted in past geological times. While the disposition 
of maximum accumulations as here bounded does not indicate 
a definite temperature zone within which petroleum has been 
formed, it does suggest a distinctly zonal distribution of petro- 
leum in which temperature may have been an important factor. 
At once the question arises as to whether these apparent 
boundaries of the zone of maximum accumulations fit well with 
the actual conditions or whether further investigation will greatly 
modify the shape and extent of the ^ important’’ areas. If there 
is this actual zonal distribution of petroleum, one must consider 
several factors involved in such limitation and with these specu- 
lations others follow such as the possibility of a ‘ ^barren’ ^ equato- 
rial belt and a productive zone in the southern hemisphere cor- 
responding with that of the northern. 
There is, of course, grave danger in assuming that the belt of 
maximum production is as observed, or that such an area may 
be expected to have anything in the way of a definite boundary, 
even theoretically. There can be little doubt, for instance, that 
this zone has offered. the most favorable conditions for explo- 
ration and it is not unlikely that with more extensive prospecting 
other great accumulations of petroleum will be found, possibly 
well outside these approximate boundaries. 
Regardless of the lack of thorough prospecting, however, there 
is reason to believe that of the three zones, the equatorial 
belt between the twentieth parallels and adjacent belts in the 
northern and southern hemispheres extendingn orth and south to 
the fiftieth parallels, the northern belt will, when investigations 
are carried to completion, be found the more productive. For 
instance, one may safely assert that, all other factors being equal, 
the amount of petroleum underlying a given area is directly pro- 
portional to the size of that area. It is evident that in the area 
of exposed lands neither the southern nor the equatorial belts 
compare favorably with the northern zone. 
Inasmuch as important accumulations of petroleum in Pre- 
Cambrian rocks are unknown, certain broad areas of Pre-Cam- 
