192 ALLUVIAL PLAIN. [CHAP. XXXIV. 



becoming always much smaller in trees of great age ; but Zuc- 

 carini, in his estimate, thinks it may be safer to assume 1-6 line 

 as the average, which would even then give the age of 3512 

 years for this single tree. 



The great number of crescent-shaped lakes to the westward of 

 the Mississippi, which formerly constituted bends in its ancient 

 channel, are also monuments of the antiquity of the great plain 

 over which the river has been wandering. Darby, the geogra 

 pher, observed that, in the steep banks of the Atchafalaya, there 

 are alternations of the bluish clay of the Mississippi arid of the 

 red ocherous earth peculiar to Red River, proving that the 

 waters of these two streams once occupied alternately consider 

 able tracts below their present point of union. ^ Since their 

 junction (an event, the date of which is unknown), the waters 

 and sediment of the Red River and Mississippi have been thor 

 oughly mixed up together, before any deposition of their mud 

 takes place in the lower country. It is evident, therefore, that, 

 when we are enabled, by geological observations such as those of 

 Darby, to distinguish the older from the newer portions, even of 

 the modem alluvial plain, we may obtain more aid in our chro 

 nological computations founded on rings of growth in buried trees ; 

 for we may then add the years deduced from stumps buried in 

 the modern parts of the delta, to those proved by the structure 

 of trees included in mud of earlier date. 



After considering the age and origin of the modem deposits of 

 the Mississippi and its tributaries, we have still to carry back 

 our thoughts to the era of the fresh-water strata seen in the bluffs 

 which bound the great valley. These, in their southern termina 

 tion, have evidently formed an ancient coast-line, beyond which 

 the modern delta has been pushed forward into the sea. Let a, 

 b (fig. 10) represent the alluvial plain of the Mississippi, bound 

 ed on its eastern side at Vicksburg, as before described, by the 

 bluffs d, at the foot of which are seen the Eocene strata, f t the 

 upper part of the bluff being composed of shelly loam, or loess, of 

 fresh- water origin, d, e (No. 2). 



At Memphis, Port Hudson, and many other places, loam of 

 * Darby s Louisiana, p. 103. 



