104 T. L. WATSON WEATHERING OF GRANITIC ROCKS OF GEORGIA *• 



the deca3'^ed rock was determined an increase over the corresponding 

 extract from the fresh rock was indicated.* 



Cause of red Color of the Soil . 



The cause of color variation in soils has been discussed in recent years 

 by Crosby ,t Dana,t Russell, § and Merrill, || Thus far the writer's obser- 

 vations are in accord with those of Crosb}^ and ^tlerrill in supposing this 

 to be a superficial phenomenon. The most brilliant hues are observed 

 to be confined to the immediate surface portions, varving from an inch 

 to many feet in depth, which gradually give wa}' in turn to more mod- 

 est hues of yellow and brown, and finally pass into the grays and other 

 colors of tlie fresh parent rock beneath, as the de})th increases. Such a 

 transition may be observed in almost any recent excavation in tbe crys- 

 talline belt of the southern Appalachians. There are, however, two 

 exceptions to this which without proper differentiation will prove very 

 misleading : 



(1) In those cases where the sections have been exposed for some 

 time the transition in vertical section becomes quickly obscured by 

 the washing down of the brilliant ochreous coloring matter from 

 aV)ove, frequently coating the section face to the entire depth of ex- 

 ])osure with a varying thickness of the ferruginous wash of appar- 

 ently the. same degree of color. This invariably happens, and may 

 be observed to advantage after heavy rains in almost any part of the 

 above belt. 



(2) In still oth(n- cases exceptions are to be found in the weathering 

 along the joint i)lanes and otber approximately vertical as well as 

 horizontal planes of weakness found cutting the rock masses. In these 

 the color is most intense along the immediate surface of the planes, 

 and extends oftentimes, in case of the vertical planes, to a considerable 

 de])th. 



Crosby is inclined to regard the increase in color of the sui)erficial 

 portions to be due wholly to dehydration of the ferric salts, while Mer- 

 rill recognizes not mereh'' the quality of the coloring matter, but empha- 

 sizes the importance oi quantity as well. Merrill •[ says: '* This is well 



*Geo. P. Merrill : Op. cit., 1897, p. 219 ; Bull. Geol. Soc. Ain., 1896, vol. 7, pp. 349-3G2. 



t W. O. Crosby : Proceedings Boston Soc. Nat. Hist., 1884-1888 (1888), vol. xxiii, pp. 219-222 ; Tech- 

 nological Quarterly, 1891, vol. iv, p. 36. 



J. I. D. Dana: Amer. Jour. Sci., 1890, xxxix, pp. 317-319. 



1 1. C. Russell : U. S. Geol. Survey, Bulletin no. 52, 1889, (;:> pp. 



II Geo. P. Merrill : Bull. Geol. Soc. Am., 1897, vol. 8, pp. 101-102; Rocks, Rock Weathering and 

 Soils, 1897, pp. 384-386. 



IJBulI. Geol. Soc. Am., 1897, vol. 8, p. 162. 



