224 THE JOURNAL OF GEOLOGY. 
iron deposits were simply layers interbedded with the associated rocks. 
The same theory of the formation of iron deposits, changed a little to 
suit local details, might consistently be applied to many iron deposits 
in the eastern states and in the Rocky Mountains, though of course the 
trough in which concentration occurred need not necessarily have been 
formed by a dike and a quartzite, as in the Penokee region. The 
trough may be formed in a great number of ways, by disturbances 
and foldings in the rocks without any dikes, by the crumbling of local 
areas of rock, etc. 
In addition to the purely geological, part of the volume, a chapter 
on previous geological work in the district and a full summary of the 
literature is given. A most excellent feature of the volume is the sys- 
tem of clear, brief summaries at the end of each chapter and the gen- 
eral outline of the volume given in the beginning, both of which are of 
the greatest value in giving a correct understanding of the subject, as 
well as a convenient means of rapid reference. 
R. A. F. PENROSE, JR. 
The Petrology of the Penokee Tron-Bearing Series. 
As an example of the value of petrographical study, both of the 
rocks of a metamorphosed series, fev se, and of the production of a 
metalliferous deposit of great economic importance, the monograph by 
Irving and Van Hise stands preéminent. Not only has the investigation 
been thoroughly and skillfully made, but the data obtained have been 
presented in such a manner as to render them accessible to those who 
may wish to follow the investigation step by step, and be able to form, 
as it were, an independent opinion. At the same time the results are 
stated in a concise form with sufficient explanation for those who do 
not care to follow the study in detail. This has been accomplished by 
placing the results in the form of general statements, and by supple- 
menting them with a tabulation of the observations. In the case of 
each formation studied the field occurrence and megascopical structure 
have been combined with the microscopical characteristics in shaping 
the history of the formation. And the probable origin of each has 
been reasonably demonstrated. 
Owing to the fullness of the petrographical portions of the report, 
which, in fact, constitute the foundation of the work, it will not be pos- 
sible to do justice to it in a brief review, in which only a few of the 
salient features can be pointed out. The most notable of these are in 
