MINERAL OIL 281 
The rock material having accumulated quickly, the organic 
matter was deposited with the shells and has been distilled 
into bitumen, which having no escape, collected in cracks 
or pores in the limestone. Gash veins of bitumen in lime- 
stone may therefore indicate no large supplies of oil, and 
boring for oil beneath a limestone in which the oil is indi- 
genous is naturally unsuccessful. Where, however, the 
limestones have been impregnated from below their bitu- 
minous veins may be a clue to underlying oil supplies. 
(4) Some mud volcanoes are due to the escape of petroleum 
vapours, which carry up with them hot mud, and pile it 
around the vent, as in the mud volcanoes of Burma, Trinidad, 
and the coast of Beluchistan. In the Baku oilfield mud 
hills thus formed are 1300 feet high. Carbon dioxide or sul- 
phuretted hydrogen denotes a volcanic origin; but a gas of 
the petroleum series indicates that the mound spring” 
is not volcanic and that petroleum may occur underground. 
{3) Burnt clay is sometimes due to the burning of petroleum, 
as in Barbados; but it may be a result of contact alteration. 
If the clay has been completely fused into a black glass 
(pseudotachylyte), the high temiperature necessary for this 
change may be due to the burning of producer gas generated 
by the action of steam upon a hot hydrocarbon. 
(6) Salt is such a frequent associate of oil that the two 
are often regarded as connected in origin. In England, 
and elsewhere, thick beds of salt are found without oil, 
but in some fields, as Ohio and Rumania, salt and oil are so 
intimately connected that the salt is believed to have helped 
in the formation of the oil. 
(7) The presence of sulphur is often also regarded as an 
indication of oil, but the association may be a coincidence. 
(8) The most likely part of a coalfield to yield oil has been 
deduced by D. White (Journ. Wash. Acad. Sci., v, 1915, 
pp. 189-212) for the Appalachian area and Mississippi valley, 
from the isovols, or lines drawn through places where the coal 
has the same ratio of fixed to volatile carbon. In an anthra- 
cite field the isovols may be above 90; some fields with 
isovols above 70 contain neither gas nor oil; a little of 
both may occur between isovols of 65-70; and oil in commer- 
cial quantities is found where the isovols are below 63. 
This method is not of universal application as some coalfields