192 EOCK DISINTEGRATION AND DECOMPOSITION 



separation of the protoxides in the form of hydrous sesqiii- 

 oxides and a general hydration of the alkaline silicates, accom- 

 panied by the formation of alkaline carbonates, which, being 

 readily soluble, are taken away nearly as fast as formed. More 

 or less silica is also removed, according to the amount of car- 

 bonic acid present, a portion of the alkalies forming soluble 

 alkaline silicates when the supply of the acid is insuiBcient to 

 take them all up in the form of carbonates. The apparent 

 anomaly here shown is partially explained by examination of 

 the various separations with the microscope. Thus the low 

 percentage of silica is found to be in large part due to the fact 

 that the residual quartz granules are, in many cases, too large 

 to pass the 120-mesh sieve, or, if passing, have been largely 

 separated in the process of washing. Further, it is found that 

 the sifting has served to concentrate the small epidotes in the 

 fine sand, and a portion of them have even come over with 

 the silt The presence of this epidote also explains in part the 

 high percentage of lime shown, since the mineral itself carries 

 some 20 to 24% of this material The large percentages of 

 magnesia, soda, and potash cannot, however, be thus accounted 

 for, and we are led to infer that the feldspathic constituents, to 

 which the alkalies are to be originally referred, have undergone 

 a mechanical splitting up rather than a chemical decomposition. 

 This view is, to a certain extent, borne out by microscopic studies, 

 but it is difficult to measure by the eye the relative abundance 

 of these constituents with sufficient accuracy to enable one to 

 form a satisfactory conclusion. The magnesia must come from 

 the shreds of mica, many of which, from their small size and 

 almost flocculent nature when decomposed, would naturally be 

 found in the silt obtained as stated. 



It is to be noted that the magnesia, together with the iron, 

 exists almost wholly in a soluble form. 



It is evident at once that we have had to do here with 

 but the preliminary stages of granitic weathering, that the 

 process is more one of disintegration than decomposition, and 

 it will be well to consider now a case in which the decom- 

 position has gone on to the condition of a residual clay, as 

 found in many of the Southern states. For this purpose a 

 biotite gneiss or gneissoid granite fround near North Garden, 

 in Albemarle County, Virginia, is selected. The temperature 

 average is here about 56.5° Fahr., with recorded extremes of 



