Let us go back, in time, to the genesis of our continent. There was once a time in the history of the earth when all the rocks were in a molten condition, and the waters of our great oceans in a state of vapor, surrounding the fiery ball. Space is intensely cold. In course of time the earth cooled off, and on the cold, solid crust geological agencies began to work. It is now conceded by the most accomplished physicists that the location of the great continents and seas was determined by the original contraction and cooling of the earth's crust; though very greatly modified by a long succession of changes, produced by the agencies of "water, air, heat, and cold," through probably a hundred million of years, until the original rock surface of the earth has been worked over to a depth of thirty or forty miles. Like human history, the events of these long _æons_ 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 Archæan--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 Palæozoic--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.