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Proceedings of the Koyal Society of Edinburgh. [Sess. 
nature of the shales, but that there is little in common with the Scottish 
shale-fields. 
The points to be noted are : — 
(1) The occurrence of shales of a cannel coal type closely associated 
with coals. 
(2) The occurrence of a more purely coal-bearing group at a higher 
horizon. 
(3) The passage of oil-shales into carbonaceous shales and impure coals. 
(4) The association of the oil-shales with impervious strata, especially 
above the seams. 
(5) The almost horizontal structure. 
(6) The occurrence of very gentle undulations where some of the richest 
shales are developed. 
(b) Evidence from Oil-Fields. 
It has been shown above that oil-shale occurs in very different environ- 
ments in different parts of the world, and that it varies also in character, 
possibly according to its environment, though retaining some characteristics 
in common which we may fairly claim to be essential to its existence. 
It now follows to describe briefly some of the essential features of oil- 
fields, to ascertain whether there are any points of resemblance between 
conditions typical of oil-fields and oil-shale fields. 
It the first place, the stratigraphical relations of coal or lignite seams to 
oil-bearing rocks may be noted. 
In any thick series of strata which contains both coal and lignite seams 
and oil rocks it is found that the upper zones are characterised by the car- 
bonaceous phase and the lower by the petroliferous phase. This fact has been 
recorded so often that it is hardly necessary to give examples, but Virginia, 
Trinidad, Burma, Venezuela, Assam, and even Baku, may be quoted as 
instances. 
It is not necessary here to enter into a discussion as to the origin of 
petroleum. It is known that oil can he formed from vegetable matter, e.g. 
from peat, and it has been suggested that kerogen has been formed from 
vegetable debris ; but from whatever raw material petroleum may be formed, 
high pressure, the sealing of deposits by impervious upper strata, fairly low 
temperature, and the presence of at least a small quantity of water are con- 
ditions that must necessarily have obtained. This is not only established 
by geological field evidence, but is generally accepted by almost every 
writer on the subject of petroleum. 
Any oil-rock to be of value must have a more or less impervious covering. 
