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Proceedings of the Royal Society of Edinburgh. [Sess. 
solidified containing it, but also a recent beach, river alluvium, or other 
superficial deposit impregnated with petroleum from underlying strata ; but 
in practice it is usual to exclude such cases and to consider an oil-rock as a 
member of a regular stratigraphical series. Be it noted, however, that it is 
the impregnation that makes the oil-rock and no other special characteristic 
of the rock itself, since the same stratum may be oil-bearing, water-bearing, 
and “ dry ” within a very short distance. 
The problem with which this paper deals may be stated shortly thus : 
“ Is there any essential relationship between oil-shales and oil-rocks as 
defined above ? ” 
Theory of the Formation of Oil- Shale. 
The distinction between oil-rocks and oil-shales set forth above is-ex- 
pressed more briefly by the statement that oil-rocks contain petroleum, and 
oil-shales contain “kerogen.” Kerogen is a term first used by Professor 
Crum Brown to denote the material which, though not being petroleum 
itself, yields petroleum on distillation. It is a very useful term, the precise 
significance of which, however, must be left to the chemist. It will be 
shown later that, even although we may not be able to state in so many 
words the actual composition or constitution of kerogen, some evidence can 
be obtained as to how it has been formed. 
In discussing the origin of oil-shales it is chiefly the origin of kerogen 
that has been considered, possibly to the exclusion of equally valuable 
evidence. 
Though no very definite theory has perhaps been formulated as to how 
the kerogen has been formed, D. R. Steuart’s work, published in Part 3 
of the Geological Survey memoir on The Oil-Shales of the Lothians, has 
been generally accepted as summing up what is known upon this subject, 
and pointing to a fairly definite conclusion, if not actually advancing 
demonstrative proof. It will suffice to quote a few paragraphs: — 
“ In the oil-shale, as in the Torbanehill mineral, paraffin and paraffin oil 
do not exist as such : they are created by the destructive distillation in the 
retorts. There is very little in shale soluble in petroleum spirit, benzene, 
carbon-disulphide, ether, and such solvents. Our substance is therefore not 
of the nature of petroleum, bitumen, or resin. All hydrogen and carbon 
compounds produced by inorganic reactions are soluble in these solvents, 
and this makes it almost certain that kerogen is of organic origin. 
“ The material has probably been deposited, together with clay, at the 
bottoms of lagoons, and has there been subjected to maceration and limited 
microbe action. Part would decompose, and only what could withstand the 
