766 THE AMERICAN NATURALIST. 
PETROGRAPHY. 
Weinschenk’s *«Die Gesteinsbildenden Mineralien ” ' affords the 
most satisfactory introduction to petrography that has yet appeared. 
Its first 47 pages are devoted to the physical and chemical methods 
employed in the separation of rock constituents and in the determi- 
nation of the character of isolated mineral particles. The remainder 
of the volume describes briefly the principal rock-forming minerals 
as they appear under the microscope. In spite of the brevity of the 
treatment necessitated by the small size of the book nearly all the 
essential facts relating to the characteristics of the individual min- 
erals are to be found in its pages. The reader is supposed to be 
already acquainted with the general principles of optics, so no space 
is devoted to this subject. 
The special feature of the book is the collection of r8 plates, 
bound in two brochures and enclosed in a pocket from which they 
may be removed for ready reference. They contain summaries of 
the physical, chemical, optical and crystallographic characters of 
the minerals discussed in the text ; a list of minerals classified accord- 
ing to their color, in thin sections, and their crystallographic habits ; 
lists of those exhibiting optical anomalies, twinning structures, dis- 
tinct cleavages, and of those whose powder reacts alkaline ; and lists 
in which the minerals are arranged according to their magnetic 
strength, their solubilities, their fusibility, their density, their refract- 
ing indices, the strength of their double refraction, and their optical 
character. ‘These tables are so well arranged and they embody 
such a mass of information in small compass that they will prove 
not only useful to younger students but also extremely valuable to 
those of maturer experience. 
So far as the writer knows, the present volume is the only modern 
elementary treatise on the microscopical characters of rock-forming 
minerals that can safely be recommended to beginners in the study, 
Ww. S. B. 
E nk, Ernst. Die Gesteinsbildenden Mineralien. | Freiburg, Herdersche 
Verlagshandlung, 1901. 146 pp., 100 figs., 18 tables. 

(Vo. 428 was mailed August 19.) 


