358 Tlic AmericiDi Geologist. Jinu-, isoo 
chemical types, no doubt there is also a constancy in their 
jjlivsical properties of which it is also necessary to take account 
in any rational classification — which, then, ought to be eclectic- 
and not based on a single idea. 
Chapter W . This contains only supplementary notes. The 
following types are characterized: notyte, kyshtinyte, absaro- 
kvte, toskanyte, ciminyte, etc. No comment is necessary on 
the diagrams of table IV. The circular diagrams are not nu- 
merous, and the rectangular diagrams are easy to understand 
at a glance ; they are very clear and represent well the chemi- 
cal composition. They are similar to those used first by Id- 
dings. 
The Diff'ereiitiatioN of M(7g/n<7s. Chapter II. 
In a rapid historical sketch the author enumerates the six 
])rincipal theories, and says that in differentiation it is necessary 
to take account of: (i.) The principle of Soret. (2.) The 
principle of the greatest work of Beithelot. (3.) The phe- 
nomena that take place in super-saturated solutions. (4.) Phe- 
nomena that take place in a mixture of liquids. (5.) The role 
of gravity. He distinguishes between dififerentiation by crys- 
tallization, and magmatic differentiation. 
Differentiation by Crystallization. 
The principal question is that of the order of succession in 
which the minerals of rocks are consolidated. At first the au- 
thor presents the historical expose of the question and a sup- 
plementary note (chap, iv ) where he tries to show an accord 
between the advocates of consolidation by order of fusibility 
and those who attach importance to the question of super-sat- 
urated solutions — since the magma can be considered either as 
a mixture in igneous fusion or as a super-saturated solu- 
tion. 
Thus, according to the author, from the order of succes- 
sion of consolidation of the minerals, there can be distinguish- 
ed three groups: (i.) Rocks in which the order of consolida- 
tion follows the rule of Rosenbusch, that is, crystallization 
commences with the most basic minerals and terminates with 
the most acid. That is the most usual case. (2.) Rocks in 
which consolidation takes place in the reverse order: The feld- 
