1915-1916.] 
45 
The Origin of Oil- Shale. 
Definitions. 
To begin with, it is necessary to define the terms which will constantly 
be used. This presents some difficulty owing to the many varieties of 
deposits which have been or are being mined and retorted for the extrac- 
tion of oil. To define an oil-shale as “any rock/ which yields petroleum on 
distillation ” is not sufficiently precise, as it would include not only oil-sands 
and the bituminous shales of oil-fields, but also nearly all coals and lignites, 
bastard coals, “rums” of the Fife coal-fields, natural asphalts, and intrusive 
bitumens of the series from gilsonite to grahamite. 
It is evident that, to exclude coals and lignites, the nature and percentage 
of the inorganic or inert portion of the deposit must be taken into account. 
Again, to exclude strata impregnated with crude petroleum, the fact must 
be emphasised that the oil cannot be obtained by trituration of the rock or 
by the action of solvents such as petrol or carbon-disulphide.* To exclude 
asphalts and solid bitumens, emphasis must be given to the fact that an oil- 
shale is a stratigraphical deposit, part of a normal geological series. 
We arrive then at a definition which, if somewhat clumsy, has at least 
the merit of being fairly precise : — An oil-sliale is an argillaceous or shaly 
deposit from which petroleum can be obtained by distillation , but not by 
trituration or treatment with solvents. 
Even with this definition the distinction between bastard coals, coaly 
shales, cannel coals and torbanite, and true oil-shales is a very fine one, 
since a series of more or less intermediate deposits can be brought forward, 
bridging the gulf between coal and oil-shale. 
In practical work, however, a rough-and-ready distinction can easily be 
made, depending on the percentage of inorganic material present: if a 
deposit difficult to classify contains too high a percentage of ash to be 
worked as a coal, and yet yields on distillation a sufficient percentage of 
oil to be worked as an oil-shale, it must be regarded as an oil-shale ; other- 
wise it may be classified as an impure coal. This still leaves the abnormal 
deposit torbanite in an equivocal position, with its average of 21 per cent, 
to 24 per cent, of ash. The true nature of this deposit will be dealt with 
specially in a later section. 
An oil-rock is much more easily defined : it is — Any rock or deposit 
impregnated with natural petroleum , which can be extracted by disinte- 
gration of the rock or by the action of solvents. This definition is so wide 
that it will include not only an intrusive igneous rock which has distilled, 
from the strata it has passed through, a proportion of mineral oil, and has 
* Broxburn shale yields only 2'04 per cent, soluble in carbon-disulphide, i.e. “bitumen.” 
