REVIEWS 489 
action of crystallization and diffusion, not accepting Becker’s conclusion 
that diffusion would proceed so slowly as to be ineffective except for very 
short distances. 
In chaps. vi-x, Harker discusses the crystallization of rock-magmas 
from their constituents on the basis of new data developed by Vogt, 
Tamman, Day, Allen, Wright, Adams, and others. He applies to the 
solution of the problems involved, the principles of physical chemistry as 
developed by Roozeboom, Ostwald, and Van’t Hoff, and concludes that 
textures are in part due to these laws, and to the relation of the actual com- 
position of the magma to that of the dominant eutectic. 
In discussion of hybridism Harker denies the importance of magmatic 
assimilation except on the smallest scale and considers hybrid rocks to 
be a minor factor in the history and development of any igneous complex. 
In his last chapter the author makes no attempt to present a new 
classification of rocks, although he says that the American quantitative 
classification marks a ‘‘retrograde movement, for here the artificial element 
is applied to the complete exclusion of the natural.”” He believes that the 
time is not yet ripe for a natural classification of igneous rocks, although 
he agrees with Becker that such a classification will probably be based 
upon the eutectics occurring in rocks, and he suggests further that it will 
involve the mode of development of various rock-types from a single 
parent magma through the action of differentiation; thus developing 
something which is comparable to the principle of descent used in the 
classification of animals and plants. 
It appears therefore, that the book is an excellent summary of our 
present knowledge, and well suited for use with advanced students of 
petrology. 
AGEN We 
Cambrian Geology and Paleontology. By CHartEs D. WaALtcortt. 
Cambrian Sections of the Cordilleran Area. From Smithsonian 
Miscellaneous Collections, Part of Vol. LIII, pp. 167-230. 
Ten plates. Washington, December 10, 1908. 
This -paper is a continuation of Dr. Walcott’s study of the Paleozoic 
rocks of western North America. The object of this preliminary correla- 
tion is to show the interrelations of the Cambrian strata and faunas in 
the Cordilleran area, particularly in California, Utah, Nevada, Montana, 
and British Columbia. Five generalized sections are described in detail 
as to character and content. There seems to be a close relationship 
