566 Reviews—Prof. Zirkel’s Petrography. 
The brilliant results achieved by the application of the micro- 
scope to the study of rocks led, at first, to the comparative neglect 
of the older methods of investigation; but the time has passed 
by when the description of the microscopic characters of isolated 
rock-specimens can be of much value. The microscope has now 
to be employed in conjunction with the chemical balance, and both 
must be brought to bear on rocks whose relations with other rocks 
have been carefully studied in the field. Petrographical students 
will, therefore, cordially welcome the appearance of a work in 
which rocks are to be treated from all points of view. Professor 
Zirkel has set himself the task of recording the present state of 
petrographical science. He has an extensive acquaintance with 
rocks, both in the field and in the laboratory, and an unrivalled 
knowledge of the literature of the subject. He is, moreover, in 
other respects eminently qualified to carry out the work he has 
taken in hand, and there can be no doubt that the present edition 
of his well known treatise will be constantly consulted by all those 
who are engaged in petrographical research. 
The first part deals with General Petrography. The methods 
employed in the investigation of rocks are first described. Then 
follows an account of the mode of development of the constituents 
and a detailed description of each of the important rock-forming 
minerals. The information on these topics is brought well up to 
date, and there are copious references to original papers. 
Rock-structures and the mode of occurrence of rocks as parts of 
the earth’s crust are next considered; and, after describing the 
various agencies by which rocks are altered, the author concludes 
the first part of his work with a broad general classification. 
As regards the technical terminology of the science, the author 
is content in most cases to accept the terms already in use. He, 
however, frequently objects to those employed by Rosenbusch. 
Thus instead of idiomorphic and allotriomorphic, which are exten- 
sively employed both in Germany and in English-speaking countries, 
he prefers the older terms automorphic and menomorphic first proposed 
by Rohrbach. He also protests strongly, both on historical and 
etymological grounds, against the use of the term granophyre in 
the sense in which it is employed by Rosenbusch. It must be 
admitted that there is great force in many of his arguments; but, 
after all, the main object of language is to convey clear and definite 
ideas, and if this result be obtained there are many who are prepared 
to tolerate a little historical injustice and even some etymological 
inaccuracy. No scientific man ever dreams of arriving at the 
meaning of a technical term by considering its etymology. 
The author separates rocks into four groups for descriptive 
purposes :— 
J. Massive eruptive rocks. 
II. Crystalline schists. 
III. Sedimentary rocks with crystalline (at least not clastic) 
structures. 
IV. Clastic rocks. 
