204 ECONOMIC GEOLOGY 
seams of sand in which the oil has collected. Adjacent wells 
pass through strikingly different beds, and usually do not in- 
terfere with one another. The oil is gradually drained from 
the porous lenticles. The chief oilfield, Yenangyaung, obtains 
its supply from an area of about 1 square mile on a broad 
flat dome. Bores opened gushing wells, but the oil is mainly 
derived from many small pumped wells, In Sumatra ang 
Java the oilfields occur among volcanic rocks, but the oil 
appears to come from Kainozoic sediments which have 
undergone mountain folding, and volcanic heat has distilled 
their organic matter into oil ; the high-grade petrol from one 
field in Sumatra for years was burnt as the cheapest way of 
getting rid of it. In Borneo within the Malayan Arc ‘and 
further from the main folding the oil occurs in Kainozoic 
rocks with a homoclinal dip. Oilfields of secondary imi- 
portance occur in China where natural gas and oil have been 
obtained from salt-wells in the Triassic and Permian red 
sandstones for salt ; some of the wells are 3000 feet deep and 
took generations to bore. 
The continents of Africa and Australia have hitherto 
yielded no important supplies of oil, except in Egypt, or 
of natural gas, though a little has been found in Queensland. 
O1L SuarLe 
Oil shale is a clay which on distillation yields petroleum 
of various grades, usually ranging from asphalt or paraffin 
through heavy lubricating and fuel oils to the lighter illuminat- 
ing oil, kerosene, and to petrol and the petrol ethers. Typical 
oil shale itself contains no oil and only a small proportion 
is soluble by the ordinary solvents for organic materials and 
hydrocarbons. Some materials, such as some Californian 
diatomaceous earth which is impregnated with oil, have 
been classified as oil shale, and according to Cunningham 
Craig oil shale is clay which has adsorbed oi] from percolating 
petroleum. True oil shale however contains no petroleum, 
and the oil obtained from it is produced by destructive dis- 
tillation of its organic constituents, The oil producing 
material is a pyro-bitumen, i.e. a material which is altered 
into bitumen by heat, 
Oil shale is classed as a sapropelic coal, as a variety of 
cannel and is classified as earthy cannel (cf. p. 266).