280 ECONOMIC GEOLOGY 
in organic material is most likely to lead to a prolific oil- 
field. Accordingly those great subsiding areas known as 
geosynclinals are the seat of the chief oilfields. The tropical 
and subtropical countries are more favourable than polar 
regions, where organic matter is less abundant. 
InpicaTioNs oF Or—The existence of subterranean oil 
is indicated by several features :— 
(1) The most obvious are seepages of gas and oil. Gas 
may escape unseen unless discharged under water; it may 
supply perpetual gas springs, such as those at Baku which 
have been burning for thousands of years in the temples of 
the Fire Worshippers. 
(2) Escaping oil often forms iridescent films on pools and 
if set on fire forms a “ burning pool.” Oil films may be dis- 
tinguished from those of oxide of iron or manganese by their 
odour, and by stirring with a stick when a metallic film breaks 
into pieces, and an oil film reunites into a single patch. A 
film of oil may be a clue to a large deep-seated supply which 
constantly renews the film as it evaporates; but it may 
result, as in some deltas from the decay of recent organic 
matter. 
(3) A petroliferous sandstone may indicate an underground 
oil supply; the rock at the outcrop may give no sign of oil, 
but freshly broken surfaces have a fefid odour, remarked 
in the name stinkstone. If petroleum or bitumen occur in 
a rock and a sample be crushed in chloroform its evaporation 
in a dish leaves a black rim. A bituminous sandstone may 
act as an impermeable cap and imprison oil in beds below. 
The Brea of California consists of pitch which has plugged 
up the outlets from underlying beds, in which the oil has 
accumulated ; the oil is reached by boring. A bituminous 
sandstone may however indicate that the oil has evaporated 
and left only a solid residue. The most famous petroli- 
ferous residues are the pitch lakes of Trinidad and Venezuela 
which have been left by the evaporation of large quantities of 
oil. Bitumen also occurs in veins and * dykes,” such as the 
albertite veins of New Brunswick, along channels by which 
oil reached the surface. Limestones often contain bitumen 
disseminated through the rock or collected in simple or 
branching gash veins. The bitumen may be the residue 
of the soft tissues of the organisms that formed the limestone.