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 limestone
 may therefore indicate no large supplies of oil, and
boring for oil beneath a limestone in which the oil is indigenous
 is naturally unsuccessful. Where, however, the
limestones have been impregnated from below their bituminous
 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 sulphuretted
 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 anthracite
 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 commercial
 quantities is found where the isovols are below 63.
This method is not of universal application as some coalfields