76 Proceedings of the Royal Society of Edinburgh. [Sess. 
of low specific gravity the water certainly floats it upward by hydrostatic 
pressure if the strata be sufficiently porous to admit of percolation ; and 
thus it is beneath impervious bands and on the crests of gentle folds or 
anticlines that the petroleum becomes concentrated, and there may finally 
be inspissated sufficiently to reach the kerogen stage. 
A clay, however absorbent, cannot contain enough light petroleum to 
furnish the material to form sufficient kerogen to make it a rich oil-shale. 
If fully impregnated with crude petroleum and subjected to weathering, 
without the possibility of drawing upon a further supply of petroleum as 
the lighter fractions are lost and dissipated, the clay will fix as kerogen a 
proportion of the hydrocarbons, but cannot become fully impregnated with 
kerogen. Should, however, crude petroleum be supplied as the weathering 
and inspissation proceed, a much greater proportion will finally be fixed as 
kerogen. This is a very simple experiment that can be made with any 
absorbent argillaceous rock. 
V. Ammonium Sulphate. 
One of the principal distinctions that has often been urged to prove 
that there is an essential difference between oil-shales and rocks containing 
petroleum is that the former on distillation yield ammonium sulphate, often 
in large quantity, and that no yield of that salt in commercial quantity is 
possible from crude petroleum. This has often been held as proof that 
crude petroleum and kerogen must be entirely different in origin. It has 
even been maintained that the percentage of nitrogen in kerogen may be 
taken as evidence that the raw material from which it has been formed 
must have been to a large extent animal as distinguished from vegetable 
matter. 
It is necessary to examine into this question thoroughly to ascertain 
whether or no such statements are justified. 
In the first place, it may be noted that coal and peat contain percentages 
of nitrogen and yield ammonium sulphate when distilled. The percentage 
of nitrogen in some Natal coals is as high as 1*5 per cent., and in some 
Welsh coals as high as 2T6 per cent., and a company has been formed in 
South Africa to utilise this nitrogen content by means of a special process. 
In Scottish oil-shales the average percentage of nitrogen is ’62 per cent., 
though running as high as ’72 per cent. In torbanite it has been estimated 
variously as from 1*53 per cent, to ’50 per cent. 
From this it is quite evident that the percentage of nitrogen in the oil- 
shale need not be considered as any evidence of origin from animal matter. 
