284 ECONOMIC GEOLOGY 
Seventh, suitable geological structures for the beds to act as 
oil reservoirs. 
Large areas of the earth can be dismissed as unlikely to 
yield oil, such as great massifs of plutonic rocks and areas of 
regional metamorphism and volcanic fields, unless they con- 
tain unaltered sedimentary rocks. 
Oil is found with different geological structures (Fig. 
63, a-j). The first favourable position discovered was along 
an anticline. The early oil wells in Pennsylvania were sunk 
at random; certain lines were productive and were called 
“directive lines.” In 1859 Sterry Hunt recognized that 
they lay along the crests of anticlinals. He concluded 
(Canad. Na. vi, 1861, PP. 242, 250-1) that the oil had been 
concentrated along these upfolds, and he remarked we 
may reasonably expect to find others (i.e. good wells) along 
the line of the anticlinal or of the folds which are subordinate 
to it.” This view had been previously suggested for Burma 
by Oldham (1855, Mission Ava, p. 309). The anticlinal 
position is favourable in water-logged beds as the oil floats 
on the water and collects beneath an impermeable arch. Any 
natural gas present will collect at the top of the arch above 
the oil. If a bore pierces the impermeable beds of such an 
anticline the gas escapes first, the oil next, and water last 
(Fig. 63 a). 
Anticlines are especially favourable reservoirs because the 
oil beds are there nearest the surface and the oil is auto- 
matically discharged by the underlying water, Hence wells 
in anticlines are so convenient and economical that they 
are naturally sought for, and prospecting for oil has been 
described as simply prospecting for anticlines. A dome 
is even more advantageous, for the oil is compressed into a 
pool at its top. 
Nevertheless, the widespread expectation of * no anti- 
clinal no oil” is not justified. Oil is often absent from 
anticlines and found in synclines. In dry rocks owing to the 
absence of water-pressure, unless local heat raises gas- 
pressure, the oil works downward and collects in the trough 
or syncline. Even where anticlines are productive oil is 
often found along the synclines. In other fields, as in the 
Argentine and Colorado, the oil is dominantly synclinal. 
Oil is also largely found in beds with a uniform or homo-