20 ECONOMIC GEOLOGY 
Many Mexican wells yielded up to 60,000 barrels a day, 
but the life of such wells is short. Heavy losses were in- 
curred by these uncontrollable flows catching fire. The 
‘“ Potero del Llano, No. 4,” 1914, which discharged an 
accumulation of oil beside a basalt intrusion into Cretaceous 
limestones, is one of those famous for the disasters due to 
its superabundant oil. 
The Isthmus or Tebasco fields on the Tehuantepec Isthmus 
were opened in 1905. The wells are associated with salt 
domes and are of the South Texas type; in the eastern part 
of the field the wells are associated with an anticline. The 
wells in the Isthmus fields have been less productive than the 
main group, but some of the oil is lighter and of better quality. 
Canapa—The oldest of the three chief Canadian oilfields 
is in the Ordovician limestones of Southern Ontario, and is 
an extension of the Appalachian field. The productive rocks 
range from the Ordovician to the Devonian and the oil 
comes from limestones ; the chief pools were in the Devonian 
Onondaga Limestone; smaller contributions come from the 
Silurian beds and the Ordovician Trenton Limestone. The 
Alberta field at the eastern foot of the Rocky Mountains is 
an extension of the Rocky Mountain field of Montana; but 
that line does not promise to extend far into Canada owing 
to the huge intrusions of granite. N.W. of Edmonton are 
the Cretaceous Athabasca tar-sands, of which the bitumen 
may be the heavy residue left by the evaporation of crude 
petroleum. The quantity of this bitumen is immense. 
Where the Arctic circle crosses the Mackenzie River to 
the W.N.W. of Fort Norman is an anticline of an Ordovician 
coral limestone, 6000 feet in thickness, and near it seepages 
of petroleum have long been known. Borings from 1914- 
21 proved the existence of oil, but the development of the 
field is hampered by its remoteness. This oilfield and one 
further north in Northern Alaska are the two farthest from 
the Equator. 
West Inpies—The West Indian Islands include four oil- 
fields. The most famous is Trinidad, to which attention 
was directed by the Pitch Lake (137 acres; 135 feet thick; 
Cadman, Tr. I. Min. Eng., xxxv, pp. 453-80), which is re- 
garded as a residual deposit left by the evaporation of 40 
million tons of petroleum (Thompson, tbid., xxxv, p. 478).