168

the coast. This pumice could hardly have come from Alaska,
or it would have been observed long before. The supposition
that it comes from the Japanese volcanoes in eruption early in
1914, though it seems far-fetched, is the only explanation that
occurs to the writer. At any rate, various materials from distant
sources find their way to the Queen Charlotte islands by ocean
currents, and it is most probable that this is the way the paraffin
came. Its occurrence in conjunction with the asphaltic bitumen
found in place in the basalts must be looked upon only as a coin-
cidence.
ACCUMULATION OF PETROLEUM.

Whatever may be the disagreement among geologists in
regard to the origin of petroleum, there is essential accord in
postulating conditions for its accumulation in bodies of commer-
cial value. These necessary geological features may be listed
under four headings.

(1). A Supply of Liquid O1l. Itis obvious, that in order
that petroleum may be extracted from the rocks by drilling,
it is necessary to have the oil in a liquid condition at the tem-
perature prevailing in the strata in which it is contained.

(2). A Porous Container. The bed or stratum in which the
oil is contained is called the ‘‘oil sand”’ irrespective of its true
lithological character. It may bea porous sandstone, a dolomitic
limestone, a layer of fractured chert or hard shale, or some other
container. It is necessary, however, that there be a consider-
able area that is porous, to allow flow of oil toward the hole.

(3). An Impervious Cover. In order that a mobile liquid
like petroleum shall not escape from the porous container, the
latter must be covered or “‘capped’”’ with some impervious bed.
This impervious cap is usually wet clay or shale, which imprisons
the oil until it is penetrated by the drill.

(4). Favourable Structure. It is conceded by everyone
that the structure of the rocks containing the oil has a most
important influence on its accumulation. There are many
different types of structure that contain oil, and the subject has
been treated at length by Clapp! and others.

1Clapp, F. G., Econ. Geol., vol. 5, 1910, p. 503.
Griswold and Munn, U.S. Geol. Surv., Bull. 318, p. 15.