The chain that extends
from Alaska to Chili within the eastern shore of the Pacific Ocean, and
the many hundreds of volcanoes of the Pacific Islands bring to the surface
vast quantities of eruptive rocks which break up and overlie the
sedimentary strata formed regularly in other ways and at other times. The
volcanoes of the Java region alone have thrown out at least 100 cubic
miles of lava, cinders, and ashes during the last 100 years--twenty times
the bulk of the materials discharged into the Gulf of Mexico by the
Mississippi River in the same period of time.
From these and similar facts, the naturalist finds how agencies of the
present construct new rocks and alter the old; and so in the light of this
knowledge, he proceeds with his task of analyzing the remote past,
confident that the same natural forces have done the work of constructing
the lower geological levels because these earlier products are similar to
those being formed to-day. After learning this much, he must immediately
undertake to arrange the strata according to their ages. This might seem a
difficult or even an impossible task, but the rocks themselves provide him
with sure guidance.
Wherever a river has graven its deep way through an area of hard rocks, as
in the case of Niagara, the walls display on their cut surfaces a series
of lines and planes showing that they are superimposed layers formed
serially by deposits that have differed some or much at different times
according to the circumstances controlling the erosion of their
constituent particles.
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