£. W. Hilgard—Lignite Beds and their Under-clays. 209 
tris), whose roots form a dense, matted mass, over a foot in 
thickness, covering the clay soil, but penetrating it very little. 
Nevertheless, as the successive annual overflows raise the level 
Arkansas and Red River. Even where the Cypress stumps are 
perfectly preserved, their roots are often seen to terminate at a 
short distance, in rather an abrupt point of heart-wood ; an 
pyent their course is scarcely, or not at all, traceable in the 
solid clay 
The cause of this complete obliteration of spongy roots or 
spongy parts of roots is doubtless to be sought in the oxidizing 
intuence of ferruginous solutions percolating from above, and 
the subsequent action of pressure upon the yielding mass. 
escape ordinary observation. If, by a i process of re- 
duction and solution, this film were also to b 
'§ Not only very often marked by minute films of (sometimes 
almost red) hydroferric oxide, but, on close examination, they 
frequently exhibit a multitude of minute “slickensides,” prov- 
ing the occurrence in the mass of internal movements, doubtless 
resulting from compression. That similar clays exhibit, at 
Petite Anse and other points, a glacier like motion down hill, I 
have elsewhere* stated. ; 
t appears to me that in these two agencies combined we 
* Smithsonian Contr. Knowl., No. 248, p. 18. 
