REVIEWS 93 
maps serve to set forth in sequence the many stages in this history, including 
that of the newly discovered Lake Arkona. 
The book by Professor Ruska of Heidelberg is an initial attempt to 
meet a like demand at the German university. Dr. Ruska has the eift of 
literary style and the ability to present his subject in attractive form with- 
out loss of scientific accuracy. Different geological formations and signifi- 
cant surface features come each in turn under discussion in connection with 
well-planned excursions from Heidelberg. No less than 138 illustrations, 
many of them original and all well chosen, make the eye the pathway to the 
mind. Professor Ruska not only knows his field, but he has shown excellent 
judgment in selecting and arranging his material. Wee hie 
Rocks and Rock Minerals. By Louis V. Prrsson, Professor of 
Physical Geology, Yale University. 12mo, pp.414. New York: 
John Wiley & Sons, 1908. 
The new petrology by Professor Pirsson is a volume whose merits are 
more fully appreciated when one considers the difficulties inherent in the 
subject, not the least of which is that of classification. If it be remembered 
that the early and simple classifications based on megascopic characters 
have gradually become more and more complicated as microscopical inves- 
tigations progressed until at present they cannot be satisfactorily used with- 
out the microscope, it may be admitted that a simplified classification for 
field work and similar uses has become extremely desirable. The classifi- 
cation adopted in the new work is essentially the same as the “‘field classi- 
fication” first proposed in connection with the Quantitative Classification 
of Igneous Rocks, of Cross, Iddings, Pirsson, and Washington in 1903. 
On this basis Pirsson has succeeded in presenting in attractive style not 
merely the major facts of petrology, but also an excellent description of 
those things which give the science life and human interest. Thus, he not 
only defines a given rock from every point of view, but he describes its mode 
of occurrence, its alteration products, its various uses, and, frequently, its 
relation to ore deposits. , 
The book is, of course, not adapted to the needs of the geologist and 
petrographer, but to those of engineering and general students whose 
knowledge of the subject need not be profound. It is arranged in three 
parts: an introductory part of twenty pages dealing with the scope, history, 
and methods of petrology, and the chemical character of the earth’s crust: 
a second part of 112 pages describing briefly the rock-forming minerals 
and giving short tables for their determination; and the main part dealing 
with igneous, sedimentary, and metamorphic rocks successively, and closing 
