52 The American Geologist. January, 1901. 
iron carbonate. This rock has been widely altered by a regional meta- 
morphism to a quartz-niagnctitc-amphibolc slate commonly known 
as actinolite schist, which has in turn been profoundly changed by the 
gabbro, the resuling rock being a coarse-grained aggregate of quartz, 
magnetite, olivine or fayalite, hypersthene, augite, hornblende etc. 
The derivation of all these minerals, including those like olivine, 
augite, and hypersthene, which are characteristic of basic igneous 
rocks and rarely found in metamorphosed sediments, from the actin- 
olite schist is shown to be possible ; and the proofs that this con- 
tact zone really belongs to the Animikie and not to the gabbro are 
summarized. The contact phenomena of the Keewatin are said to 
vary greatly with biotite as a prominent feature. In the Archaean, the 
acid rocks or granites have escaped sensible metamorphism, while the 
basic rocks or greenstones, including gabbros, diabases and diorytes, 
have been profoundly altered, and usually in a way to make them dif- 
ficult to distinguish from the gabbro itself. w. 0. c. 
Suggestions Regarding the Classification of tlie Igneous Rocks. By 
William H. Hobcs. (Jour. Geol.,8, 1-31.) 
After noting the importance of adapting the classification to the 
n^feds of the field geologist as well as the petrologist, the bearing of 
recent petrographical studies on rock classification, the definition of a 
rock as an object rather than as an integral part of the earth's crust, 
the importance of texture as a basis of classification, the need of com- 
bining chemical and mineralogical classifications as a basis for rock 
classification and of substituting quantitative for qualitative analyses, 
that rock relationships should be indicated by the combination of 
names into a binomial, or, of necessary, a polynomial nomenclature, 
the author introduces graphical methods as essential to a comprehen- 
sive study of rock analyses. The system of diagrams proposed by 
Brogger for this purpose is preferred. In these are set off on radius 
vectors the amounts of the eight principal chemical constituents reck- 
oned in molecular ratios, ferrous and ferric iron being .entered upon 
the same radius vector, and silica, because so much in excess of the 
others, being evenly divided between the two horizontal radius vectors. 
A broken line joining the intercepts on the eight radius vectors forms 
a polygon, which may be long and narrow, or short and thick, convex 
above or below, or re-entrant in anj^ portion, left or right-handed etc.. 
according to the chemical constitution of the rock. When viewed in 
this diagram the rock comes to have a handwriting by which it may be 
instantly recognized ; and when drawn to scale the diagram not only 
shows the chemical character of the rock but all the results of analysis 
may be read from it numerically. The main purpose of this paper is 
to adapt the Brogger diagram to represent, not merely an individual 
analysis, but a rock species or type covering a considerable range of 
differing analyses. In other words, composite diagrams are proposed, 
each representing the average of a group of analyses. Examples are 
given for the principal types of plutonic and volcanic rocks. The pos- 
