80 
Proceedings of the Koyal Society of Edinburgh. [Sess. 
determining the state of inspissation of the material that we know as 
kerogen. In the Scottish oil-shales the percentage of sulphur averages 
about 1*4, and, though it doubtless exists partly as sulphides and sulphates 
apart from the kerogen, the latter must contain, according to Dr Mills, a 
considerable portion of the sulphur, since he found '49 of sulphur in a total 
of 3622 per cent, of organic constituents of average good Scottish oil-shale. 
The concentration of sulphur in the inspissated products of petroleum 
is too well known to require description ; but as oils vary greatly in sulphur 
percentage, some being almost free from it, it is obvious that the proportions 
of sulphur in different inspissated oils and oil-rocks will also show a 
considerable range of variation. 
Thus analyses of the various products into which we can divide Trinidad 
asphalt, itself formed from a very sulphurous petroleum, show for sulphur 
a gradation as in the case of nitrogen compounds. 
In the 
Malthenes. 
In Total 
Bitumen. 
In the 
Asphaltenes. 
In Organic 
Matter not 
Bitumen. 
Per cent. 
2*9 
Per cent. 
6T6 
Per cent. 
10-9 
Per cent. 
10*32 
In the solid native bitumen we find that gilsonite, the least inspissated, 
contains T79 percent.; grahamite a percentage varying from *93 to 179, 
the samples being taken from different districts; and albertite T06-T20. 
VI. Conclusions. 
To sum up the evidence brought forward is perhaps hardly necessary, 
but it will be as well to state the conclusions arrived at, and to apply them 
to the shale-fields that have been briefly described : — 
(1) Kerogen is formed by the inspissation of petroleum. 
(2) During the inspissation the nitrogen and sulphur compounds 
become concentrated in the most inspissated or weathered products. 
(3) At a certain stage of inspissation, which is reached gradually, the 
organic matter becomes insoluble in carbon-disulphide, and thus ceases to 
be classed as a bitumen. 
(4) Ceteris paribus, the state of inspissation increases with age. 
(5) An oil-shale is formed by the power of certain clays or shales of 
absorbing and adsorbing inspissated petroleum, particularly unsaturated 
hydrocarbons. 
(6) This power apparently depends on the colloid contents of the 
argillaceous rock. 
o 
