REPORT ON THE GEOLOGY OF NORTH AMERICA. V 



diluvial sand and gravel which conceals all the other forma- 

 tions of the country; a circumstance which will enable us to 

 distinguish between them and another group of more ancient 

 alluvial deposits to be described further on. 



Recent Changes in the Mississippi. — I am informed by Mr. 

 Tanner, the geographer, that a striking example of the manner 

 in which this river ordinarily varies its channels was witnessed 

 about two years ago, at the mouth of the Red River. A re- 

 markable bend at that place, known as one of the longest and 

 most circuitous loops in the Mississippi, was cut off by the 

 simple expedient of digging a very short trench across the 

 narrow neck which the stream was daily scooping away. In 

 24 hours steam-boats passed through the new channel, and it 

 immediately became the outlet of the Red River, which before 

 entered the Mississippi by the lower side of the bend, but now 

 discharges itself along the upper. By this change the river has 

 been shortened 20 miles. 



When it is recollected that in freshets the current of the Mis- 

 sissippi descends at the rate of five and even six miles an hour, 

 and at low water at the rate of two miles, it will at once be seen 

 how great a load of sedimentary matter it can annually sweep 

 down into its delta, and how rapidly this must augment both in 

 height and superficial area. As an example of the rate at which 

 it is growing, the Old Balize, a post erected by the French about 

 the year 1724, at the very mouth of the river, is now two miles 

 above it. There was not at that time the smallest appearance 

 of the island, on which, 42 years after, UUoa caused barracks 

 to be erected for the pilots, and which is now known as the 

 New Balize. 



The distance from the mouth of the river, at which the chief 

 deposit of sediment usually takes place, is about two miles. 

 When these shoals accumulate sufficiently, they form small 

 islands, which soon unite and reach the continent; and thus 

 the delta increases. So enormous has been the growth of such 

 deposits, not only opposite the mouths of the Mississippi, but 

 around the whole northern shore of the gulf, that nearly the 

 entire coast of Louisiana is inaccessible, from the shallowness 

 of the water, except immediately through the channels of the 

 rivers. 



An almost universal feature in the entrances of the rivers of 

 the Atlantic is the bar obstructing their mouths. That of the 

 principal entrance of the Mississippi had, in 1722, about 25 feet 

 of water upon it; Ulloa, in 1767, found 20 feet at the highest 

 flood; and in 1826 the depth was only 16 feet. 



Above these obstructions the rivers are generally much 



