368 The American Geologist. December, i8y9 
With reference to the District granites Prof. Merrill* fur- 
ther says: "Aside from its state of disintegration the newly- 
formed soil differs from the massive rock mainly in that its 
feldspathic and other silicate constituents have undergone a 
certain amount of hydration." A mechanical analysis of the 
residual sand from these rocks shows no clay, but the smallest 
grade material found was silt with a diameter in millemeters 
of o. lof, of which there was 4.25 per cent present, correspond- 
ing in the above table to grade 6, and there listed as very fine 
sand. 
In the case of a micaceous gneiss from Albemarle county, 
Virginia, Dr. Merrill;); found a total loss based on chemical 
analyses of the entire rock, of 44.67 per cent, corresponding 
to 35.73 per cent of clay on mechanical analyses, in the decayed 
portion of rock. Clearly, this represents a case chiefly of 
chemical decomposition. 
The difference as brought out by the results m the case 
of the adjacent dikes of the Virginia diabase for the same local- 
ity is believed to be essentially one of difference in degree of 
decay. In each case the iron is the constant factor, and the 
Al-O-i indicates a loss, with approximately the same propor- 
tional loss for the other constituents. In the case of the to- 
tal loss for the entire rock 70.31%, the final limit had been ap- 
proximately reached, where all of the iron had, in the presence 
of an abundant supply of oxygen, become completely oxidized, 
and, therefore, retained at the expense of a proportionate loss 
in the remaining prominent constituents ; while this limit has 
not been so nearly reached in case of the adjacent dike where 
the total loss amounts to only 37.10 per cent. This is made 
the more significant when the difference in the iron and alum- 
ina contents for the fresh and decomposed rock in the two 
cases is noted. 
Fresh rock Soil 
I§ II I§ II 
Ai.^Og 13.48—23.58 1 3. 19—32.05 
FegOa 11.60 — 6.85 35-69 — 9.83 
*Loc. cit. 
t Bulletin U. S. G. S. 1898, No. 150, p. 378. 
t Merrill, G. P., Bull. Geol. Soc. Am. 1897, 8, 159, 160. 
§ Olivine diabase. Loc. cit. 
