81 
1915-16.] The Origin of Oil-Shale. 
(7) The argillaceous rock impregnated with petroleum residues is 
enabled to retain them long after porous sandstones in the vicinity have 
lost all traces of petroleum by weathering and lixiviation. 
To apply these conclusions to the Scottish oil-shale fields is very 
simple, and it is possible to give a sketch of the geological history of the 
late- and post-Carboniferous period with special reference to the formation 
and preservation of the shale-fields. 
By the close of the deposition of the Coal Measures the petroliferous 
stage had been reached in the Lower Carboniferous rocks, since in favour- 
able circumstances a pressure of from 150 to 200 atmospheres is, so far as 
we know, ample to ensure the formation of petroleum. 
The petroleum formed would be concentrated in the most porous 
strata, the great freestones of the Oil-Shale group, though doubtless impreg- 
nating every band of sufficient porosity. 
Great earth movements set in at the close of the period and threw 
the strata into numerous synclines and anticlines, towards the latter of 
which the petroleum would naturally migrate under gas-pressure and 
hydrostatic pressure. 
One of these great anticlines we see in the Pentland Hills, and there 
is distinct evidence on the map (fig. 2) of a much smaller and less important 
but parallel anticline running from the south of Dechmont, north of 
Pumpherston, and through Broxburn to Kirkliston. 
Denudation of the anticlines commenced immediately, and proceeded 
so far that our Carboniferous strata are now found in great basins, while 
the intervening anticlines have been entirely removed, and with them 
almost all traces of the petroliferous phase. In only one region in the 
Carboniferous territory the lower groups remained in minor anticlines un- 
denuded. Here the earth movement, acting in a slightly different direction, 
produced the secondary anticlines of Dechmont, Pumpherston, and Kirkliston. 
In these folds, and in the minor wrinkles of Dalmahoy, Straiton, and Carlops, 
the petroleum driven out by the incursion of water made its last stand. 
As time went on, faulting supervened and still more complicated the 
structure. The faults are admittedly later than the folds, and the very fact 
of their formation shows that the load was becoming lighter under relentless 
denudation. The petroleum then came under the influence of weathering 
processes, which in favourable circumstances affect oil to a very great depth. 
Inspissation and all that it implies — loss of lighter fractions, formation of un- 
saturated hydrocarbons and oxidation — began and is still proceeding. Thus 
the petroleum would gradually become richer in unsaturated hydrocarbons, 
in sulphur and nitrogen compounds and complex oxidised molecules. 
VOL. xxxvi. 6 
