PJUXC/PLES OF ROCK IVEATHERIXG 863 



scope shows it to be composed mainly of hornblende and soda- 

 lime feldspars with interstitial areas of titanic iron. The clay or 

 soil to which it gives rise on decomposing is deep brownish-red 

 in color and highly plastic, though distinctly gritty from the 

 presence of undecomposcd feldspars. Though so rich in iron it 

 is to be noted that the residual clay is little if any deeper in color 

 than that from the gneiss. The analysis shows that 37.56 per 

 cent, of the total rock has disappeared. 



The limestone of the fourth series is of Carboniferous Age, 

 very impure, crystalline granular, and of a dark chocolate-brown 

 color. The residual clay from its decomposition is a trifle darker, 

 highly plastic and quite impervious. The analyses, calculations, 

 and descriptions are from the work of Penrose.^ 



It is to be noted that all that the lime which existed as car- 

 bonate, has been entirely removed, as shown by the absence of 

 carbonic its acid in the clay. Farther, that the clay, notwith- 

 standing highly hydrated condition, in reality contains scarcely 

 half the amount of water it would, had the small amount (2.26 

 per cent.) in the fresh limestone been allowed to accumulate 

 without loss. 



DISCUSSION OF RESULTS AND RESUME. 



Taking now into consideration in connection with these 

 analyses the statements embodied in the first part of this paper 

 relative to the agencies of degeneration, and making due allow- 

 ance for possible errors in our methods of calculation, there are 

 certain general deductions that may, apparently, be drawn with 

 safety. In the resume given below, however, reliance is placed 

 not more upon our own analyses than upon results obtained by 

 others as given in existing literature.'' 



Let us briefly review the subject and make the deductions 

 accordingly. 



In glancing over the columns of our analyses it is at once 



'Ann. Rep. Geol., Survey of Arkansas, 1890, Volume I. 



'See especially Roth's Allegemeine u. Chemische Geologie, Vol. Ill, and Ebel- 

 man's papers in Annales des Mines, Vols. VII, 1845, and XII, 1847. 



