No. 419.] NOTES AND LITERATURE. 947 
The Calculation of Rock Analyses. — Now that so much interest 
centers about the chemical composition of rock magmas and the rep- 
resentation of their composition in terms of molecular ratios, a recent 
paper by Kemp! on the methods of calculating rock analyses in these 
terms will be of great use to all students of rocks. In this paper 
the author shows how to transform percentages into molecular pro- 
portions, and from these how to calculate the mineral composition of 
any given rock. The most valuable portion of the article is a series 
of tables in which the * molecular proportions " of each of the rock- 
forming oxides is indicated for its corresponding “ percentage "M 
rock analyses. 
Weathering of Granites. — The conclusions drawn by Watson ° 
from the results of an interesting study of the weathering of a num- 
ber of granitic rocks of Georgia are as follows : Assuming that Fe:0; 
has remained constant, (1) the amount of water in the weathered 
rocks increases rapidly as decomposition advances. At the same 
time there is a loss of SiO, and of all the metallic oxides except 
ALO; which in some cases shows a relative increase. (2) The loss 
of SiO, is not caused by solution of the quartz of the original rocks, 
but is the result of the decomposition of silicates. (3) CaO and 
Na,O disappeared in larger quantities than MgO and K,O. (4) The 
total loss of constituents varies between 7.68 % in weathered phases 
to 71.82 % in thoroughly decomposed forms. 
Origin of Phenocrysts in Granites. — The same author 4 has also 
investigated these granites with respect to the origin of their pheno- 
crysts. He describes in detail a large number of occurrences and 
concludes his study in these words : * The absence of (a) definite 
arrangement or orientation among the phenocrysts; 
crysts from the border zones of the massif — grada . 
interior porphyritic facies peripherally into an even granular granite 
of coarse texture and the same mineral an 
(€) the further absence of evidence of magmatic resorption oe 
rosion of the phenocrysts ; and (7) the presence or abundant "e 
sions of all the ground-mass constituents characterizing the pepe 
tabular phenocrysts of the Georgia porphyritic granites, fully yustuy 
the conclusion that the phenocrysts in these rocks were formed 77 
Plac, and are not intratelluric in origin." - 
tion from an 
1 School of Mines Quart., vol. xxii, p- 75: 
2 Bull. Geol. Soc. Amer., vol. xii (1901), P- 93- 
3 Journal of Geology, vol. ix (1901), P- 97- 
