392 SECTIONAL TRANSACTIONS.—C. 
importance of the governing factors which control such concentration, such as 
gravity, hydrostatic pressure, gas pressure, capillarity, etc., is still, however, a matter 
of dispute in different fields. All these factors doubtless operate in inducing migration 
in varying degree. An arrest of the slow migration of the oil, whether upward in wet 
rocks or downward in dry rocks, and brought about by geological structure or 
diminution in porosity, tends to form oil pools. It is practically certain that Salt 
Domes and Plugs act in arresting and accumulating oil, and are not the source or 
carriers of oil in quantity. 
The problem of the origin of oil, and its occurrence in some geological systems, 
as in the Tertiary, in great pools in many parts of the world, while similar strata in 
other systems are barren of oil, is still unsolved or in serious question. 
The weight of evidence is in favour of an organic origin, and from minute organisms, 
such as Diatoms, Foraminifera and others destitute of hard parts. Shallow water 
marine deposits with such abundant organic remains, and with intercalated porous 
beds capable of acting as reservoirs, are all potentially oil-bearing. 
It is likely that the necessary conditions for the decomposition of the organic 
matter and generation of oil are deposition in stagnant waters, and the sealing up of 
the sediment to permit of the action of anerobic bacteria. 
In some cases the oil may have been deposited with clay in shallow marine waters, 
the oil having previously been formed by the decomposition of vegetable matter in 
swamps and lagoons, and afterwards carried by streams to the sea, as first suggested 
by Murray Stuart for the Pegu series of Burma. 
The most abundant supplies of oil occur in Tertiary shallow water marine strata 
which fringe the great geosynclines of the Old and New Worlds, and flank the chief 
mountain ranges. 
The fact that Tertiary strata contain most of the known oil supplies may be 
partly explained by their recent age, the oil not having had time up to the present to 
escape in sufficient quantity. The presence of oil pools in some older systems, e¢.g. 
the Mississippian of North America, may be due to their great lateral extent and 
comparative quietude since the deposition of their sediments, together with their 
effective cap rocks. 
Is it possible that in Carboniferous and Cretaceo-Tertiary periods the food supply 
for oil-generating organisms derived from excessive vegetable growth on the lands 
was more plentiful than in other periods ? 
Dr. G. M. Less. 
Bituminous rocks, oil shales or traces of oil are not uncommon in many countries 
at one or perhaps several horizons throughout the sedimentary succession ; but the 
formation of oil in quantity, and its subsequent concentration into oil pools of com- 
mercial value, requires a set of conditions which is undoubtedly exceptional. Oil 
may be carried by later migration, whether in vertical or horizontal direction, far 
from its original source rocks, and in consequence it is often difficult to obtain 
unequivocal evidence regarding the oil mother rock and the nature of the source 
material. 
Oil appears to have originated throughout long eras of geological time under various 
conditions of sedimentation, and probably no single source material was responsible 
for all the oil. The source material for coals of different type was equally varied, 
though all coals have one fact in common, namely, a vegetable origin. Most oils, 
though not necessarily all, have been formed in rocks of marine or estuarine origin. 
There is no known proved case of oil as such being formed at the present day, but oil 
can be obtained by distillation from many marine muds or oozes. Investigation of 
the bottom of the Black Sea has proved the presence of yellow and white ether soluble 
fatty bodies in the mud at a depth of 920 metres. 
The original source material of present-day oil underwent a ‘ cracking’ process 
during the immense course of time when it lay buried to a depth of many thousand 
feet under later sediments. In many cases this depth is from 10,000 to 20,000 feet, 
or exceptionally even more. This represents approximately as many pounds per sq. 
ete as feet of overburden for average sediments and a temperature of 150° 
to : 
Oil is associated in many countries with salt and gypsum deposits, but the writer 
knows of no case of oil in quantity occurring in such rocks where there is not a 
probability of its having its origin in normal marine rocks. Algz flourish in many 
