EXTENT AND CHARACTER OF THE EROSION. 225 



which prevail on the Cumberland plateau. The coarse conglomerate 

 serves as a reservoir from which the plants derive sufficient moisture 

 to enable them to flourish even in verjr dry weather. Although this 

 growth crowns a cliff with a vertical face of over 100 feet and this cliff 

 in turn forms the crest of a gorge 1,000 feet in depth, still the upper sur- 

 face of the conglomerate presents essentially swamp conditions, and the 

 waters which trickle over the rocky face of the cliff are doubtless heavily 

 charged with the products of plant decay. 



The geodes found by the writer in Overton county, Tennessee, though 

 possibly composed of a more soluble form of silica, also show that a sol- 

 vent is present at the surface of the earth in ordinary atmospheric con- 

 ditions which operates readily upon this form of silica. 



Conclusions respecting the Extent and Character of Erosion at 



, Baselevel. 



Altogether it seems to the writer that under certain conditions — con- 

 ditions which are doubtless present on a peneplain approaching base- 

 level — the various forms of silica which make up the major portion of 

 the rocks are soluble, and therefore are removed in a manner similar to 

 the removal of lime and magnesia, except that the operation is much 

 slower. 



The sharp differentiation between the floor and the slopes of the ob- 

 served basins indicates that practically all of the waste has been removed, 

 but silica constitutes only a portion of this waste; therefore it seems 

 highly probable that the remaining portion, consisting chiefly of the 

 silicates of alumina, has also been acted upon chemically and carried off 

 by the sluggish streams draining the basins. These residual compounds 

 of alumina are usually in a more finely divided state than silica; conse- 

 quently the streams may continue to transport them long after they have 

 ceased to carry the larger grains of silica, and also after they have prac- 

 tically reached baselevel; but there will still be a portion, and perhaps 

 a large one, which cannot be accounted for by this process, but which 

 has disappeared, and such disappearance can only be explained by the 

 theory of chemical action. If the effect of chemical action is noticeable 

 in the small basins in West Virginia, how much more pronounced must 

 its effects have been on the extensive peneplains of Cretaceous and Ter- 

 tiary ages. 



The facts herein presented, if correctly interpreted, indicate relatively 

 intense chemical action on a surface at or near baselevel, and therefore 

 have a direct bearing on the subject of land sculpture. The present 



