1U8 



atmosphere. By this expression, I do not mean to de- 

 ny that some of the simple component parts of rocks, 

 are sometimes, when exposed to waters filtrating 

 through the great mass, found in a decomposed state ; 

 such may have been the case of kaolin, mica, and a 

 number of other mineral substances ; but not being 

 subject to the operations of wind, rains, &c. afford 

 little or no addition or increase, to the subjacent dis- 

 tricts, nor to alluvial formations. Among the second 

 class of rocks, or those of the transition kind, we find 

 that there are but two species which are peculiarly 

 liable to decomposition ; (viz.) The porous, or imper- 

 fectly formed trap, and Gray Wack. The aggregate 

 proportion which these bear, to that of the general 

 mass of rocks that are presented to view upon our 

 globe, is comparatively small ; and although at first 

 view we should be inclined to believe, from their struc- 

 ture, that they are liable in an eminent degree, to decom- 

 position, it will be found, on a close examination 

 of the several circumstances necessary to be taken into 

 view, in order to ascertain the fact, that the progress of 

 decomposition, even of these rocks, is not so extensive 

 and so rapid, as is generally believed. 



Among rocks of the third class, or those of secondary 

 formation, and also those of the alluvial kind, which 

 I am disposed to consider as of a ternary formation, 

 the several kinds of red sand- stone, and the sand or 

 free stone of mote recent formation, are those most sub- 

 ject to decomposition. Of these the former do not con- 

 stitute a very large proportion in the scale of rocks, 



