^ GEOLOGY. 803 



In the former report, based on the best procurable data, on this 

 subject, your committee deduced the annual quantity of water dis- 

 charged by the Mississippi river to be only 8-91 parts of the rain- 

 water falling in the whole valley ; consequently, a disposition of 8-91 

 parts must be made by its evaporation. That report suggested the 

 progressive increase of the evaporating process as consequent from 

 the rapid denudation of the primeval forests, and consequent expo- 

 sure of the lands to the action of the sun and winds, Avhich are so ob- 

 viously well calculated to cause a very considerable increase of evapo- 

 ration of the falling waters by dissipating the aqueous matter in the 

 atmosphere of the valley. Any such progressive increase in the evap- 

 orating process must necessarily, to some certain extent, leave the 

 river less and less water to discharge. 



These inferences may not seem to be well supported by facts, 

 during the several years that have intervened between the time of 

 submitting the last and the preseut report; for the annual swelling of 

 the waters in several of these years would seem to imply but little 

 diminution in the quantity of water transported to the ocean, and 

 but little reliance on the facts and inferences as detailed in that 

 report. 



But our subsequent observations have been directed to the facts 

 and phases of another very important element in the same connection, 

 which, in that report was in a great measure overlooked ; or, at least, 

 received less attention than its importance demanded, as having, and 

 still continuing to have, no inconsiderable effect in both the physical 

 and commercial relations, as affecting navigation, and, therefore, now 

 requiring special notice. 



This feature, in this connection, consists in the very considerable 

 change now in progress with respect to the sedimentary matter with 

 which the waters of the rivers are charged on their descent to the 

 ocean. When the valley of the Mississippi was an uncultivated waste, 

 and its lands clothed in their primeval forests with the waters of 

 the river annually overflowing all its low lands these waters could 

 not have had the same proportional quantity of sedimentary matter in 

 suspension during floods they now have ; because this dense and un- 

 broken vegetation must have been a great protection against the 

 corroding effects of falling and running waters, so that much less 

 must have been washed into the river than at the present time ; and even 

 the most of that must in time of high- water, have been deposited on the 

 banks and in the woods, into which the waters were constantly run- 

 ning on their way to the ocean. Thus, the waters of the river, in 

 periods of inundation, from its source to its mouth, would let fall its 

 sand and sedimentary matter all along in the bottoms and on the 

 banks, and would necessarily have that much less to urge into the 

 ocean ; so that the bar at the river's mouth, during high water, would 

 not be subject to interpose such obstructions as at the present impede 

 navigation. 



But, during the last few years, this system of things has been 

 undergoing rapid changes, inasmuch as the face of the country is 



27 



