No. 413.] REVIEWS OF RECENT LITERATURE. 427 
Graphic Representation of Rock-Analyses. — Miiggs! proposes 
a scheme for the graphic representation of the chemical composition 
of rocks based on Brógger's and Michel-Lévy's scheme. The rela- 
tive percentages of the various metallic constituents are indicated by 
means of polygons drawn through points plotted on eight radii. 
Within this is a second polygon which represents the silica content. 
The size of the latter is determined by the percentage of this constit- 
uent present, and the relative sizes of this polygon and the outer 
one are an indication of the rock’s acidity. In constructing the inner 
polygon the percentage of silica present is divided into eight equal 
parts, and each is plotted in one of each of the radii. In plotting 
for the outer polygon the Al,O; is divided into three parts determined 
by the proportion borne by K,O and Na,O to one another and the 
other bases. 
The Origin of the Glaucophane-Schists. — Rosenbusch, as is well 
known, has hitherto suspected that true glaucophane-schists are 
genetically associated with sedimentary rather than with igneous 
rocks, but so few analyses of these schists have been made that the 
supposition has not been capable of chemical investigation. In a 
recent? article, however, he shows that some of the schists have the 
composition of a normal gabbro magma. In these epidote, zoisite, 
lawsonite, prehnite, margasite, and garnet are usually if not always 
present. Rocks of this kind are closely related to amphibolites. 
Other glaucophane-schists he still believes to be metamorphosed 
sediments, but analyses of these are lacking. 
Washington? supplements Rosenbusch's investigations in an arti- 
cle in which he records and compares fifteen analyses of these 
Schists. Upon comparing their analyses he discovers that the rocks 
fall into two main groups, a very basic group with a content of 
SiO, varying between 46% and 49.7% and a very acid group with 
SiO, between 74.5% and 82.5%. The former he believes, with 
Rosenbusch, to be derived from gabbros, diabases, or their tuffs. 
The acid glaucophane-schists he thinks are derived from cherts, 
quartzose shales, or quartzites. The basic forms scarcely differ from 
the amphibolites in chemical composition, the formation of the 
one or the other kind of schists depending probably upon conditions 
of metamorphism. 
1 Neues Jahrb. f. Min. etc., Bd. i (1900), p. 100. 
2 Sitzb. kin. preuss. Ak. Wiss. Berlin, Bd. xlv (1898), p. 716. 
3 Amer. Journ. Sci., vol. xi (1901), p. 35. 
