gether spoken of as
representing Paleozoic time. William Smith's system of strata, next
above these, once called "secondary," represents Mesozoic time, or
the age of reptiles. Still higher, or more recent, are Cuvier and
Brongniart's tertiary rocks, representing the age of mammals. Lastly,
the most recent formations, dating back, however, to a period far enough
from recent in any but a geological sense, are classed as quaternary,
representing the age of man.

It must not be supposed, however, that the successive "ages" of the
geologist are shut off from one another in any such arbitrary way as
this verbal classification might seem to suggest. In point of fact,
these "ages" have no better warrant for existence than have the
"centuries" and the "weeks" of every-day computation. They are
convenient, and they may even stand for local divisions in the strata,
but they are bounded by no actual gaps in the sweep of terrestrial
events.

Moreover, it must be understood that the "ages" of different continents,
though described under the same name, are not necessarily of exact
contemporaneity. There is no sure test available by which it could be
shown that the Devonian age, for instance, as outlined in the strata of
Europe, did not begin millions of years earlier or later than the period
whose records are said to represent the Devonian age in America. In
attempting to decide such details as this, mineralogical data fail us
utterly. Even in rocks of adjoining regions identity of structure is no
proof of contemporaneous origin; for the veritable substance of the
rock of one age is ground up to build the rocks of subsequent ages.
Furthermore, in seas where conditions change but little the same form
of rock may be made age after age. It is believed that chalk-beds still
forming in some of our present seas may form one continuous mass dating
back to earliest geologic ages. On the other hand, rocks different in
character maybe formed at the same time in regions not far apart--say
a sandstone along shore, a coral limestone farther seaward, and a
chalk-bed beyond. This continuous stratum, broken in the process of
upheaval, might seem the record of three different epochs.

Paleontology, of course, supplies far better chronological tests, but
even these have their limitations. There has been no time since rocks
now in existence were formed, if ever, when the earth had a uniform
climate and a single undiversified fauna over its entire lan