he statement that half
the work of organic advance had been accomplished at the beginning of
the Cambrian rocks. The writer is of the opinion that the development
which took place before that age must have required a much longer
period than has elapsed from that epoch to the present day. We thus
come to the conclusion that the measurement of duration afforded by
organic life indicates a yet more lengthened claim of events, and
demands more time than appears to be required for the formation of the
stratified rocks.

The index of duration afforded by the organic series is probably more
trustworthy than that which is found in the sedimentary strata, and
this for the reason that the records of those strata have been
subjected to numerous and immeasurable breaks, while the development
of organic life has of necessity been perfectly continuous. The one
record can at any point be broken without interrupting the sequences;
the other does not admit of any breaches in the continuity.

THE MOON.

Set over against the earth--related to, yet contrasted with it in many
ways--the moon offers a most profitable object to the student of
geology. He should often turn to it for those lessons which will be
briefly noted.

In the beginning of their mutual history the materials of earth and
moon doubtless formed one vaporous body which had been parted from the
concentrating mass of the sun in the manner noted in the sketch of
the history of the solar system. After the earth-moon body had
gathered into a nebulous sphere, it is most likely that a ring
resembling that still existing about Saturn was formed about the
earth, which in time consolidated into the satellite. Thenceforth the
two bodies were parted, except for the gravitative attraction which
impelled them to revolve about their common centre of gravity, and
except for the light and heat they might exchange with one another.

The first stages after the parting of the spheres of earth and moon
appear to have been essentially the same in each body. Concentrating
upon their centres, they became in time fluid by heat; further on,
they entered the rigid state--in a word, they froze--at least in their
outer parts. At this point in their existence their histories utterly
diverge; or rather, we may say, the development of the earth continued
in a vast unfolding, while that of the moon appears to have been
absolutely arrested in ways which we will now describ