MINERAL OIL 205 
Ordinary oil shale has a tough leathery texture with some- 
times a satiny lustre. It resists weathering, so that it often 
stands out on cliff faces while adjacent shales break down 
into mud. It may be cut by a knife like tobacco, and 
varieties rich in oil when cut curl before the knife and are 
therefore known as curly shale. The richer varieties can be 
ignited by a match and burn freely. Microscopic examina- 
tion shows that the oil shale contains many vegetable re- 
mains, spores, grains of wax and resin; the characteristic oil 
producing constituents are yellow bodies which have been 
identified as pollen grains, alge, or spores. In many cases 
they are secondary bodies, often with a radial structure 
formed by the redeposition of cellulosic and resinous material. 
The working of oil shale founded the mineral oil industry. 
In 1847 a small seepage of petroleum in the Riddings Colliery 
in Derbyshire was leased by James Young but the supply 
soon failed. Young concluded that the oil must have been 
distilled from a shale, and on heating this material in a re- 
tort similar oil was produced. The largest supply of oil by 
distillation was obtained from torbanite; the first oil works 
was erected at Bathgate, and treated the torbanite which 
yielded 120 gallons to the ton. 
Young's patents were applied in America and many oil 
shale works established ; they were afterwards used as re- 
fineries for the oil from wells. The Scottish work had been 
preceded by some working of shale at Autun in Southern 
France. The Torbane Hill mineral was soon exhausted, 
and oil shales used instead; the richer seams were worked 
out until in later years the Scottish industry was maintained 
on shale which yielded less than 20 gallons of oil to the ton. 
After the opening of the oil wells in America the working of 
oil shale would have become economically impossible, but 
for the yield of paraffin wax and sulphate of ammonia, which 
have at times been more profitable than the oil. The ammonia 
is formed from the nitrogen in the shale. 
The Scottish oil shales occur in the Lower Carboniferous 
rocks E. of Edinburgh, and are interbedded in a series of 
sandstones, clays, and limestones, which include about twenty 
seams of worked oil shale. The beds appear to have been 
laid down in lagoons containing abundant fossil fish and 
entomostraca ; their tissues and the debris of plants which