79 
1915-16.] The Origin of Oil-Shale. 
If any further proof be required that the kerogen of the Scottish 
shale-fields is a highly inspissated and therefore oxidised product, it is 
given in an analysis by Dr Mills (. Destructive Distillation) of the kerogen 
of good average shale. Omitting the sulphur and nitrogen, there is a 
percentage of oxygen as high as 16'33, the analysis showing carbon 73’05, 
hydrogen 1062, and oxygen 16’33. 
Thus the evidence from nitrogen contents and yield of ammonium 
sulphate bears out what we have already seen to be the case with the 
hydrocarbons forming kerogen ; if the latter be derived from inspissated 
petroleum, and concentrated and fixed in the shale by inspissation and 
adsorption, the nitrogen compounds from which the ammonia is distilled 
have also been concentrated in the same manner, and in favourable circum- 
stances to an extent even greater than in the case of the majority of 
Scottish shales. 
Thus the argument that the yield of ammonium sulphate from oil-shales 
shows that they are in no way connected with petroleum, not only falls to 
the ground, but proves to be a very strong argument in favour of the 
kerogen having been derived from petroleum. 
In this connection it may be recalled that many bituminous shales have 
been called oil-shales and yet do not yield ammonium sulphate in com- 
mercial quantities. Of these the Colorado “ oil-shales ” and the Kimmeridge 
“ oil -shale ” may be mentioned. Both are really shales impregnated with 
petroleum, and have not yet reached the stage of true kerogen-bearing 
shales.* Such shales are liable to be ignited spontaneously at outcrop, 
when they may burn and smoulder for long periods, a phenomenon never 
observed in the case of a true kerogen shale, with the exception of one 
recorded case in New South Wales. Similar burnt shales are frequent in 
Trinidad, where they are called “ porcellanites ” ; they mark a transition 
stage between the lignitic and the petroliferous phase, but are also 
associated with oil-rocks. They occur frequently among argillaceous beds 
a short distance above lignite seams, and have in many cases been leaf- 
beds. On the Red Deer River in Alberta similar naturally burnt clays may 
be seen not far above coal-seams. In Barbados at Burnt Cliff, and near 
the asphalt lake in Trinidad, similar burnt clays may be seen; in the 
latter cases it is quite evident that the material burnt is petroleum. 
The well-known burning of the Kimmeridge Clay points to the material 
burnt being of a similar nature, bituminous rather than kerogen. 
The concentration of sulphur compounds affords another means of 
* “Oil-Shales of N.W. Colorado and N.E. Utah,” by E. G. Woodruff and David T. Day, 
Bulletin 581a, U.S. Geol. Survey. 
