Their thickness, as we know them, is estimated at 50,000
feet; a thickness which must represent many millions of years.
But we do not know how much thicker than this they may be. They
underlie the oldest rocks that have ever been exposed to the gaze
of the geologist. They include sedimentary deposits, showing the
action of water, and even probable traces of organic remains, but
they are, especially in their deeper and older sections,
predominantly volcanic. They evince what we may call a volcanic
age in the early story of the planet.
But before we pursue this part of the story further we must
interpolate a remarkable event in the record--the birth of the
moon. It is now generally believed, on a theory elaborated by Sir
G. Darwin, that when the formation of the crust had reached a
certain depth--something over thirty miles, it is calculated--it
parted with a mass of matter, which became the moon. The size of
our moon, in comparison with the earth, is so exceptional among
the satellites which attend the planets of our solar system that
it is assigned an exceptional origin. It is calculated that at
that time the earth turned on its axis in the space of four or
five hours, instead of twenty-four.
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