raised that the geologist does not find
any rocks which he can identify as portions of the primitive crust
of the earth. It seems to me that it would be too much to expect the
survival at the surface of any part of the first scum that cooled on
that fiery ocean. It is more natural to suppose that millions of years
of volcanic activity on a prodigious scale would characterise this
early stage, and the "primitive crust" would be buried in fragments, or
dissolved again, under deep seas of lava. Now, this is precisely what we
find, The oldest rocks known to the geologist--the Archaean rocks--are
overwhelmingly volcanic, especially in their lower part. 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. We have already seen that the tidal
influence of the sun has the effect of moderating the rotation of the
planets. Now, this very rapid rotation of a liquid mass, with a thin
crust, would (together with the instability occasioned by its cooling)
cause it to bulge at the equator. The bulge would increase until the
earth became a pear-shaped body. The small end of the pear would draw
further and further away from the rest--as a drop of water does on the
mouth of a tap--and at last the whole mass (some 5,000,000,000 cubic
miles of matter) was broken off, and began to pursue an independent
o