No. 380.] REVIEWS OF RECENT LITERATURE. 613 ; 
Rhodolite, a New Variety of Garnet.1— Under this name a vari- 
ety of garnet is described which occurs in placer deposits in Macon 
County, North Carolina, and is notable for its fine amethystine and 
rose color, and gem quality of clearness. It occurs only in rolled or 
etched fragments, together with the following minerals, in more or 
less abundance: quartz, pyrope, corundum, spinel, iolite, cyanite, 
fibrolite, hornblende, staurolite, rutile, chromite, monazite, zircon, 
gold, sperrylite, menaccanite, and bronzite. 
The specific gravity of the material which was very free from 
inclusions was 3.838. Chemical composition : 
SiO, Al,0O; Fe,0, FeO MgO CaO Sum 
41.59 23-13 1.90 15.55 17.23 0.92 100.32 
corresponding to the formula 2Mg; Al. (SiO,);.Fe;Al. eae a mix- 
ture of two pyrope molecules with one of almandine 
PETROGRAPHY. 
Classification of Igneous Rocks. — In an interesting paper on the 
relation between the chemical and mineral composition of igneous 
rocks, Iddings* shows very plainly that the mineral composition of a 
cooling magma is dependent both on the original composition of this 
magma and upon the character of the minerals that rst separate 
from it. It is well known that quartz is usually associative only with 
the polysilicate-feldspathic minerals. Of these minerals the most 
acid one possible with the available silica in the magma, is that 
which forms first. The alkalies seem to control an equal amount of 
alumina, forming alkali-feldspathic molecules, the alumina in excess 
combining with calcium to form anorthite, or with magnesium and 
iron to produce the amphiboloids. These and several other laws 
less firmly established have been carefully worked out by comparing 
the mass composition of massive rocks with their mineral composi- 
tion. The author discusses in detail the mineral composition of 
magmas (1) in which the alkali is wholly soda and in which alu- 
mina is present in equivalent quantities with the soda ; (2) those in 
which the sole alkali is potash with alumina in equal quantity ; and 
(3) those in which the alkalies control an equal amount of alumina 
and in which lime and additional alumina occur in the proportion of 
1 Hidden, W. E., and Pratt, J. H. Amer. Journ. of Sci., vol. clv, p. 294, 1898. 
? Journ. of Geol., vol. vi, p. 219 
