53 
1915-16.] The Origin of Oil-Shale. 
it from any other Carboniferous tract in Scotland” (the italics are the 
writer’s). 
In England, where the Lower Carboniferous rocks are as a rule of marine 
rather than estuarine and deltaic type, oil-shales are absent. The signifi- 
cance of these facts will appear later. 
As regards the oil-shale itself, it varies somewhat in physical character. 
It is described in the Geological Survey memoir as “ a fine black or 
brownish clay shale, with certain special features which enable it to be 
easily distinguished in the field.” The chief of these characteristics are a 
brown streak and a toughness and resistance to disintegration by the 
weather. It is distinguished from ordinary carbonaceous shale — known by 
the miners as “ blaes ” — by the latter being far heavier, brittle, and often 
gritty, while it disintegrates into fragments when exposed and finally 
“ reverts to its original condition of clay or mud.” 
A good oil-shale, on the other hand, resembles “ hard dark wood or dry 
leather,” and it can be “ cut or curled up with the edge of a sharp knife.” 
It is “free from grittiness, and is often flexible as well as tough.” “In 
internal structure oil-shale is minutely laminated.” 
A distinction is sometimes drawn between “plain” and “curly” shales, 
the latter being contorted or curled and glossy on the surface of bedding 
planes, while the former variety is flat and smooth. There does not seem, 
however, to be any essential difference between the plain and curly 
varieties, and a seam occasionally consists partly of plain and partly of 
curly shale. 
These descriptive notes are taken from the Geological Survey memoir 
— the recognised work on Scottish oil-shales. 
The most significant point with regard to oil -shale is that it may 
graduate upwards or downwards into bituminous “blaes” and ordinary 
“blaes” so insensibly that it may be impossible to draw a dividing line 
between them. Shale seams also, when traced for a distance, may pass 
into ordinary carbonaceous shales or blaes, and thick seams may contain 
bands of barren blaes or ribs of hard calcareous ironstone or cementstone. 
The roof of a good shale seam is usually impervious blaes, though lime- 
stone, sandstones, and “ fakes ” (sandy shales) may occur as the floor. As 
a general rule, however, the oil-shale seams are associated with argillaceous 
rocks, and may be considered as a special modification of the prevailing 
shaly type of sediment. 
Another very significant point of which mention is made in the 
