MINERAL OIL 
279 
ents of ordinary animals and plants. True oil shale contains 
no oil, but organic residues which, when heated in a retort, 
are converted into oil. According to the organic theory 
mineral oil is similarly produced from the organic matter in 
sedimentary deposits. Oil comes from beds charged with 
organic material and is only found in igneous rocks which have 
been injected with it from sedimentary rocks. Pockets and 
trickles of petroleum are frequently met with in the igneous 
rocks of the Scottish oil-shale field, but only where the oil 
may have been distilled out of shales; the field of Bacur- 
anao in Cuba is fed from an serpentine, which has doubtless 
been impregnated from sediments. 
That the mineral oil was not formed synthetically is shown 
by its optical properties. Synthetic oil does not cause 
circular polarization, but mineral oil does. Its circular 
polarization is sometimes attributed to the presence of some 
organic oil—cholesterol (Ca5H,40), which is familiar as 
lanoline and is an animal product, or phytosterol, a corre- 
sponding oil derived from plants. According to this im- 
probable suggestion mineral oil is partly inorganic and partly 
organic. 
Most authorities agree that mineral oil is of organic origin, 
but there is wide difference of opinion as to whether it is 
mainly vegetable or animal. Its frequent association with 
coal and lake deposits is advanced in support of its vege- 
table origin; but large quantities come from Silurian and 
Ordovician rocks that are earlier than any land vegetation 
that would have produced spores and seeds. Animal 
tissues can be distilled into oil similar to petroleum, and most 
of the organic matter in many oil-producing beds is more 
likely to be animal than vegetable. The Scottish oil shales 
contain abundant fossil fish and entomostraca, and many 
fields obtain their oil from shales rich in foraminifera and 
other animals. The constant association of fish with oil 
deposits has indeed led J. H. Macfarlane to claim in a volume 
entitled Fishes, the Source of Petroleum (1923, p. 414), not 
only that petroleum is wholly of animal origin, but that 
fish alone are its source. Petroleum is probably derived from 
both plants and animals,"and animals appear to contribute 
the larger share. 
The quick accumulation of thick masses of sediment rich