85 
1915-16.] The Origin of Oil-Shale. 
them with coals or intrusive bitumen rather than shales. The field 
evidence proving their occurrence in regular beds makes it practically 
certain that they must be looked upon as very highly bituminous coals, 
indicating the approach to the oil-bearing stage even in beds of what 
is now coal ; this is confirmed by the association with oil-shales in 
both cases. 
Applying the conclusions detailed above to the shale-fields of South 
Africa, it is obvious that we are dealing with the overlapping of the 
petroliferous and carbonaceous phases. Probably the coal-bearing stage 
had been reached before pressure was sufficient to initiate oil-formation. 
No flexures exist to have had the effect of concentrating petroleum towards 
any particular localities, and consequently such organic matter as survived 
to reach the petroliferous stage is distributed very widely, the oil-shales do 
not show sudden changes in character and yield, and are nearly all inclining 
to a carbonaceous character and association with coal seams. 
In New South Wales somewhat similar conditions obtain, and coals and 
oil-shale are very intimately associated. But the petroliferous phase has 
undoubtedly been reached even in some deposits which from their very low 
inorganic contents must be now classified as coals. 
Very gentle undulations have certainly caused a concentration of what 
is now kerogen towards certain localities where the richest seams are 
worked. As a whole, however, New South Wales crystallises for us the 
transition stage : with greater pressure earlier applied, more pronounced 
earth movement, and effective sealing by impervious strata the territory 
would have been an oil-field. With less pressure more slowly applied the 
carbonaceous phase would be even more pronounced and oil-shales would 
be absent altogether. 
It is perhaps unnecessary to add further instances which can be gleaned 
from many parts of the world where thick series of strata containing 
vegetable organic matter may be studied. The field evidence is fairly 
complete, and it is borne out by chemical and experimental work, so far 
as if has been undertaken. The idea is not new that something approach- 
ing to an oil-shale can be made artificially by the “ affinity ” of fuller’s 
earth for the unsaturated hydrocarbons in natural petroleum,* and had 
experiments been made with more highly inspissated material more striking 
and suggestive results would doubtless have been obtained. Field evidence 
gives the key to the conditions that must be applied in such experimental 
research. 
* Oil-Shales of the Lothians , p. 162 . 
