MINERAL OIL 285 
clinal dip (cf. Fig. 63, k), which is also described by the 
hybrid term of uniclinal. Many of the chief oilfields are in 
homoclinal and not anticlinal areas. The oil may rise along 
a porous bed in the homoclinal series and collect where the 
bed is blocked by a fault or dyke, or becomes thinner or 
denser, or has been plugged by bituminous matter. 
Oil also occurs in horizontal beds. It has been claimed that 
the oil really occurs along anticlines which are so gentle 
that they are not recognizable; but if the beds were anti- 
clinal, the pools in the successive beds should occur one 
below another, like the saddle-lodes of a goldfield; but 
the pools at different levels may not be superposed (Fig. 
63, ¢). The pools may be due to lenticles or patches of 
porous sand in clays, or the concentration of the oil by sur- 
face tension. If a mixture of oil and water soaked into a 
bed of irregularly mixed sand and clay, the surface tension 
would force the oil into the sandier patches, leaving the 
water in the clays. Many oil pools in horizontal and inclined 
beds are doubtless due to this process. 
Oil may also occur in thin seams or gash veins in limestone, 
due to the gradual production of bitumen from the organic 
matter of the rock, and its collection in shrinkage cracks. 
Large accumulations of eil also form where fractured lime- 
stones are capped by an impermeable bed, as in Mexico; 
but in these cases as there may be no evidence on the surface 
as to the distribution of the fissures, boring is very uncertain. 
Though oil is not formed in igneous rocks it may be forced 
into them by gas-pressure when distilled from adjacent 
rocks by heat. Colossal oil pools occur in Mexico beside 
intrusive igneous rocks (Fig. 63, 7). 
Estimation oF Or REsourceEs—The estimation of the re- 
serves of oil in a field is more difficult than that of coal or 
ore. It is rarely possible for a surveyor to enter the oil 
bed, while an adequate number of bores to test the reserves 
might be too costly and even dangerous. Of the two chief 
methods of estimating oil reserves, the first is determination 
of the capacity of the oil sands, i.e. their area multiplied by 
their thickness and by the amount of pore space. The 
result shows the maximum amount of oil that would be pre- 
sent if the whole bed were saturated with oil. This “ satur- 
ation method " is deceptive because the pores may be partly