165

translucent container. A. A. MacPhail of the B.C. Oilfields
showed one of these agates to the writer, and said that they were
occasionally found at Tian point, though considered more or
less of a rarity. |

Figure 23 is a generalized diagram illustrating the various
ways in which the tarry matter is found.

Se

a
—_——_! .
& Lz a &
‘  EKE®) -
e- £ Sepa &
&

ree pti. ic ak: ne

Geological Survey, Canada.

Figure 23. Generalized diagram illustrating the mode of occurrence of tar
in chalcedony-quartz amygdules and veins in basalt.

Origin. It is not proposed to give here an account of the
theories that have been advanced to explain the origin of petro-
leum. Excellent summaries with abundant references are given
by Clarke! and H6fer.? In general it may be said, that petroleum

1 Clark, F. W., U.S. Geol. Surv., Bull. 491, pp. 681-704.
2 Hofer, Hans. Das Erd6l, Leipzic, 1906, pp. 160-229.

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