150 FRESH-WATER LOAM OF NATCHEZ. [CHAP. XXXI. 



shells, of which we collected twenty species, are all specifically 

 identical with those now inhabiting the valley of the Mississippi. 

 The resemblance of this loam to that fluviatile silt of the val 

 ley of the Rhine, between Cologne and Basle, which is generally 

 called &quot; loess&quot; and &quot; lehm&quot; in Alsace, is most perfect. In both 

 countries the genera of shells are the same, and as, in the ancient 

 alluvium of the Rhine, the loam sometimes passes into a lacus 

 trine deposit containing shells of the genera Lymnea, Planorbis, 

 and Cydas, so I found at Washington, about seven miles inland, 

 or eastward from Natchez, a similar passage of the American 

 loam into a deposit evidently formed in a pond or lake. It con 

 sisted of marl containing shells of Lymnea, Planorbis, Paludiiia, 

 PUysa, and Cyclas, specifically agreeing with testacea now 

 inhabiting the United States. With the land-shells before men 

 tioned are found, at different depths in the loam, the remains of 

 the mastodon ; and in clay, immediately under the loam, and 

 above the sand and gravel, entire skeletons have been met with 

 of the megalonyx, associated with the bones of the horse, bear, 

 stag, ox, and other quadrupeds, for the most part, if not all, of 

 extinct species. This great loamy formation, with terrestrial 

 and fresh-water shells, extends horizontally for about twelve 

 miles inland, or eastward from the river, forming a platform 

 about 200 feet high above the great plain of the Mississippi. 

 In consequence, however, of the incoherent arid destructible 

 nature of the sandy clay, every streamlet flowing over what 

 must originally have been a level table-land, has cut out for 

 itself, in its way to the Mississippi, a deep gully or ravine. This 

 excavating process has, of late years, proceeded with accelerated 

 speed, especially in the course of the last thirty or thirty-five 

 years. Some attribute the increased erosive action to partial 

 clearings of the native forest, a cause of which the power has 

 been remarkably displayed, as before stated, within the last 

 twenty years, in Georgia.* Others refer the change mainly to 

 the effects of the great earthquake of New Madrid, in 181112, 

 by which this region was much fissured, ponds being dried up 

 and many landslips caused. 



* See tonte, p. 29. 



