86 PETROLOGICAL ABSTRACTS AND REVIEWS 
part of the subject, say the authors, would involve the consideration of 
a wide range of conditions entering into sedimentation, such as is beyond 
the scope of the book. Twenty-two pages are devoted to primary 
gneisses and schists; the term primary gneisses being used for banded 
igneous rocks whose banding was produced during the consolidation 
of the rocks (ortho-gneisses, in part), and metamorphic gneisses for those 
that were formed by anamorphic processes (para-gneisses, in part). 
Among the criteria given for distinguishing primary from metamorphic 
gneisses are field relations, textures and structures, and mineral and 
chemical compositions. Instructive triangular diagrams are given 
showing the igneous or sedimentary characteristics of a rock. 
A number of chapters are devoted to ocean, lake, river, and under- 
ground solutions as by-products of the metamorphic cycle; and there 
is a discussion of the metamorphic cycle as a basis for the genetic classi- 
fication of commercial mineral products. 
The last chapter of the third part deals with the net results of the 
metamorphic cycle, and the answer to the question, “Is the metamorphic 
cycle closed?” is that “adequate evidence of it is lacking . . . . such 
evidence as there is points rather toward the incompleteness of the 
cycle. 
The fourth part, on laboratory work in metamorphism, is one of the 
most useful and instructive in the book. It treats of the megascopic 
and microscopic study of specimens and the measurement of specific 
gravity and porosity, a plate being given for calculating the porosity 
from the moisture of saturation and the specific gravity of the rock 
materials. It shows how analyses may be compared to determine the 
relative and absolute gains and losses of constituents, and gives instruc- 
tions for the use of various straight-line and circular diagrams. There 
are instructions for comparing analyses by means of rectilinear co- 
ordinates, and for constructing triangular diagrams of various kinds. 
The determination of mineral compositions of rocks from their chemical 
analyses by recalculation or by the mineral slide-rule, and the calcu- 
lation of volume- and energy-changes are treated, and finally there are 
suggestions for laboratory study. 
The book is strongly recommended, not only to students of meta- 
morphic geology, but to all students of petrology and to advanced 
students in economic or general geology. The authors are to be con- 
gratulated on having presented the subject in a clear, simple, and, at 
the same time, most interesting way. 
