86 Proceedings of the Royal Society of Edinburgh. [Sess. 
It must be left to the chemist to determine what reactions, if any, take 
place between inspissated petroleum residues, or their unsaturated hydro- 
carbons, and the colloid contents of an argillaceous rock. Whether com- 
pounds insoluble in carbon-bisulphide are formed with bases, and whether 
the nitrogen, sulphur, and oxygen compounds play any essential role in the 
process, may require much elaborate research which is out of the domain 
of the geologist. 
The practical result, as we have seen, is summed up in the formation of 
kerogen ; and in searching for kerogen-bearing strata we must proceed even 
as in the search for petroleum, by giving special attention to anticlinal 
structures in the particular stratigraphical sequence to be prospected. 
It is claimed, then, that, in light of the field and laboratory evidence set 
forth above, the theory of the origin of an oil-shale quoted in the earlier 
part of this paper is, to say the least, inadequate ; that oil-shale fields and 
oil-fields are not separate and distinct phenomena but inextricably con- 
nected ; that an oil-shale field is but the evidence of a former petroliferous 
phase ; and incidentally that Scotland still possesses the relics of a once 
great and valuable oil-field, which has passed, like the presumed great coal- 
fields of Ireland, before man appeared on earth to benefit by it. 
(. Issued separately May 16, 1916.) 
