Like human history, the events of these long _aeons_ are divided into
periods. The geologist divides the past history of the earth and its
inhabitants into five Great Times; and these, again, into ages, periods,
epochs, and eras.

At the close of the first Great Time--called Archaean--the continent
south of the region of the great lakes, excepting a few islands, was
still submerged beneath a shallow sea, and therefore no portion of the
Mississippi was yet in existence. At the close of the second great
geological Time--the Palaeozoic--the American continent had emerged
sufficiently from the ocean bed to permit the flow of the Ohio, and of
the Mississippi, above the mouth of the former river, although they were
not yet united.

Throughout the third great geological Time--the Mesozoic--these rivers
grew in importance, and the lowest portions of the Missouri began to
form a tributary of some size. Still the Ohio had not united with the
Mississippi, and both of these rivers emptied into an arm of the Mexican
Gulf, which then reached to a short distance above what is now their
junction.

In point of time, the Ohio is probably older than the Mississippi, but
the latter river grew and eventually absorbed the Ohio as a tributary.

In the early part of the fourth great geological Time--the
Cenozoic--nearly the whole continent was above water. Still the Gulf of
Mexico covered a considerable portion of the extreme Southern States,
and one of its bays extended as far north as the mouth of the Ohio,
which had not yet become a tributary of the Mississippi. The Missouri
throughout its entire length was at this time a flowing river.