48 
Proceedings of the Royal Society of Edinburgh. [Sess. 
group, in the oil-shale province in South Africa, and in the neighbourhood 
of the New South Wales oil-shales. The suggestion, however, only carries 
the difficulty a step further. Such natural distillations are well known 
locally, but the concentration of kerogen into seams or shale-beds has to 
be accounted for as well as the bitumen in the original steams of coal or 
shale. A more widespread action than any local distillation is required 
to have caused the great development of oil-shales as known and worked 
in Scotland. 
Professor Quekett, in discussing the origin of torbanite, made one 
pregnant suggestion in his conclusion that vegetable fossils in the seams 
of oil-shale or Torbanehill mineral are “ accidental,” and not necessarily 
connected with the presence of kerogen. 
II. Field Evidence. 
(a) From Oil-Sliale Fields. 
It is necessary now to consider all relevant geological field evidence 
to ascertain whether any widespread conditions may be taken as favourable 
to the occurrence of oil-shales, to treat oil-shales not necessarily as separate 
phenomena sui generis, but to learn whether there are any essential associa- 
tions or modes of occurrence which are typical of oil-shale fields. 
Most of our knowledge of oil-shales is derived, perhaps unfortunately, 
from Scotland. Unfortunately, because in the Scottish shale-fields there 
may be many conditions which do not obtain in other countries where oil- 
shales exist, and we may be in danger of regarding as essential matters 
which are purely accidental, if we only study the Scottish, fields. 
1. The Lothians . — The oil-shales of Scotland occur in the Carboniferous 
formation, and more especially in that division of it known as the 
Calciferous Sandstone series. 
Thin oil-shales are first seen in the Wardie shales, where there are also 
thin and unworkable seams of coal. As we ascend in the series, oil-shales 
become more frequent and thicker, but workable coals do not begin to 
appear till towards the top of the Oil-Shale group. Above this group, 
though coals become much more frequent, shale seams are fewer and poorer 
and generally unworkable ; while in the Coal Measures proper, though a few 
thin oil-shales are known, they are not worked, either on account of poor- 
ness or because they are not thick enough. Thus, taking the formation 
from the horizon where workable oil-shales first appear, we have a shale- 
bearing group with occasional coals passing up into a coal-bearing group 
with shales dying out completely towards the top. In other words, dealing 
