162 G. P. MERRILL — WEATHERING OF MICACEOUS GNEISS. 



be remembered that the color imparted to a bed of sand or clay depends 

 not merely upon the quality of the coloring constituent, but also upon 

 its quantity. This is well shown in the fractional separations made by 

 washing these residual cla} 7 s and sands, whereby they are separated into 

 proportional parts of varying degrees of fineness. Thus the disintegrated 

 diabase of Medford, Massachusetts, described by the present writer at a 

 previous meeting,* is of but a dull brown-gray color, yet the finest silt 

 separated out by washing, and which is but a ferruginous clay, is of a 

 distinct brown-red color. A larger amount of this fine material would 

 naturally impart a more decided brown-red color to the residual sand. 

 Now, rock-weathering in warm, moist climates in particular, is accom- 

 panied by a leaching process, as I have elsewhere pointed out, f whereby 

 the more soluble constituents are gradually removed. But, of all those 

 constituents occurring in essential quantities in ordinary rocks the ferric 

 salts and the alumina are most refractory; hence the final product of 

 decomposition is a highly ferruginous clay — a clay composed essentially 

 of a mechanical admixture of hydrous aluminous silicates, free silica, and 

 free sesquioxides of iron — and its color is due to the preponderance of 

 this free oxide which has become segregated through the leaching out of 

 the more soluble constituents. While not taking exception to the view 

 advanced by Professor Crosby to the effect that the iron in these south- 

 ern soils may exist in a state of partial dehydration, I would account in 

 part for the brilliancy of color merely through the presence of this excess 

 of coloring matter. Color is then indicative of advanced decomposition 

 and, other things being equal, of geological antiquity. This view is ren- 

 dered the more plausible when it is recalled that the post-Cretaceous 

 decay of the granites of the District of Columbia, to which I have pre- 

 viously called attention, has in most cases given rise to residual sands 

 and clays of but a gray-brown color, while rocks of a precisely similar 

 nature, decaying under the same climatic conditions, but so situated that 

 the products of decay have been allowed to accumulate for longer periods, 

 give rise to colors of characteristic brilliancy. 



Zeolites in the fresh Rock and their possible Origin. 



The occurrence of zeolites in the still fresh rod?: and the proportion- 

 ately large amount of soluble matter in both the rock and the residual 

 soil brings up for consideration the probable form in which these soluble 

 constituents exist. Those at all conversant with the literature of the 

 subject will at once recall the work of Lemberg, Hilgard, and others and 

 the conclusions reached regarding the formation of zeolites during the 



*Bull. Geol. Soc. Am., vol. 7, 1896. 



f Rocks, Rockweathering, and Soils. The Macmillan Company, New York, 1897. 



