The length of the Mississippi, from the Gulf of Mexico, pursuing its involutions, may be stated to be three thousand miles. By estimates from the best sources made, respectively, during the expeditions of 1820 and 1832, it is shown to have a winding thread of three thousand one hundred and sixty miles. Taking the barometrical height of Itasca Lake at fifteen hundred and seventy-five feet, it has a mean descent of a fraction over six inches per mile. As one of the most striking epochs in American geography, we have known this river, computing from the era of Marquette's discovery to the present day (July 13, 1832), but one hundred and fifty-nine years--a short period, indeed! How rich a portion of the geology of the globe lies buried in the flora and fauna of the tertiary, the middle or secondary, and the palæozoic eras of its valley, we have hardly begun to inquire. It will, _doubtless_, and, so far as we know, _does_, contribute evidences to the antiquity and mutations of the earth's surface, conformably to the progress of discoveries in other parts of the globe. The immense basins of coal, found in the middle and lower parts of its valley, prove the same gigantic epoch of its flora which has been established for the coal measures of Europe,[167] and sweep to the winds the jejune theory that the continent arose from a chaotic state, at a period a whit less remote than the other quarters of the globe. While the large bones of its later eras, found imbedded in its unconsolidated strata, prove how large a portion of its fauna were involved in the gigantic and monster-period. [167] Entire trees are often found imbedded in its rocks of the middle era, as is evidenced by an individual of the juglans nigra, of at least fifty feet long, in the River De Plaine, valley of the Illinois. _Vide_ Appendix. CHAPTER XXIV. Descent of the west, or Itascan branch--Kakabikoñs Falls--Junction of the Chemaun, Peniddiwin, or De Soto, and Allenoga Rivers--Return to Cass Lake. Itasca Lake lies in latitude twenty-five seconds only south of Leech Lake, and five minutes and eleven seconds west of the ultimate northerly point of the Mississippi, on the Queen Anne summit; it is a fraction over twelve minutes southwest of Cass Lake. The distance from the latter point, at which discovery rested in 1820, is, agreeably to the observations of Lieutenant Allen, one hundred and sixty-four miles.