368 The Amcncan Geologist. Jun.-, is9s* 
As to the specially descriptive part of the work, that is suf- 
ficiently presented in the French resume added to the book 
along with the general conclusions. In this part are rock de- 
scriptions which are not of much interest. He offers also some 
considerations on dynamo-metamorphism, but nothing new. 
These are given simply for the purpose of explaining what i? 
his point of view. Having first been an adept disciple and 
servant of Rosenbusch he has later changed his mind. 
Concluding Remarks by the reviewer. 
The revicwei does not at all judge of the descriptive por- 
tion of this work, not being acquainted with the petrography of 
the Caucasus. He simply remarks that of 404 pages 150 are 
occupied with the descriptive portion. The theoretical por- 
tion greatly preponderates. Of this part Chapters HI and IV 
are not resumed. 
In chapter II, on differentiation, there seems not to be any 
new ideas, as Lacroix has already said for the portion con- 
cerning magmatic differentiation. 
As to chapter I on classification — here the idea of the co- 
eflficient of acidity («')was applied before the author by 
Schrockenstein ; the discussion of the relations between the ox- 
ides, R-0:RO, and Xai:0:K:;0, has been presented before by 
Lang, Iddings, etc.; the considerations on the oxides in func- 
tion of the content of SiO: were introduced by Rosenbusch and 
others (Lapparent, Geologic, p. 600) ;the curves of tables I-IIl 
were constructed according to the curves published by Id- 
dings; Lewinson-Lessing says (p. 242) that the work of Id- 
dings in which are found these diagrams and conclusions simi- 
lar to his own ("Origin of Igneous rocks"), was unknown to 
him, for, in spite of all his efforts, he was only able to obtain 
the book when his own was already in process of publication. 
Xow, it is to be noted that the book of Lewinson-Lessing, ac- 
cording to his statement in the preface, was printed at the com- 
mencement of the year 1897. and that of Iddings in 1892. 
Further, the author cites this same book often, and even an- 
tagonizes Iddings. And then, a professor at a university not 
being able, in spite of all his efforts, to obtain a book in five 
vears! This may be taken as an index of the value of this 
work. 
