September 18, 1908] 



SCIENCE 



375 



by a simple chemical test. The first division 

 is made on the basis of differences of grain 

 (granularity) into three classes. These are 

 divided according as the rock in question 

 is porphyritic or non-porphyritic. Further 

 division is made, chiefly upon differences 

 of mineral composition, into a few broad 

 groups which are designated by familiar 

 rock names (granite, diorite, etc.) firmly 

 fixed in petrological literature by long usage. 

 The more important varieties falling under 

 these various groups or kinds of rock, but 

 which for microscopical or other reasons have 

 received special names from petrographers, 

 are referred to in appropriate places, so that 

 the significance of such names when encoun- 

 tered can be easily found by reference to the 

 book. Amiong the names adopted for the 

 rock groups, it may be noted that dolerite has 

 been retained, very wisely it seems, to desig- 

 nate those granular, igneous rocks, with pre- 

 dominant ferro-magnesian minerals but in 

 which it is not possible to tell positively just 

 what ferro-magnesian mineral is present. 

 This group will therefore include much that 

 is commonly referred to as diabase. Felsite 

 (resp. felsite-porphyry) is used for those 

 nearly or entirely dense rocks, light in color, 

 and generally highly feldspathic in character, 

 while basalt (resp. basalt-porphyry) covers 

 those dense, dark-colored, igneous rocks, for 

 the most part ferro-magnesian in character. 

 It will be noted that both of these terms, fel- 

 site and basalt, have been given a broader 

 meaning than is customary in books cover- 

 ing a somewhat similar field. For example, 

 dacites and most andesites fall under the head 

 of felsite. Directions, however, for recogniz- 

 ing such varieties when it is possible to do so 

 macroscopically, are given. The classifica- 

 tion is summarized in a convenient table on 

 page 202. 



While some teachers will doubtless wish to 

 make some minor additions to the scheme of 

 classification in their class-room work, the 

 scheme as a whole appears so sane, simple 

 and practical that it can not fail to meet with 

 general approval, and it is to be hoped that it 

 will also come shortly into general use. In 



such an event it would be a strong influence 

 toward uniformity and simplicity of practise 

 among field workers in geology and petrology. 



In Chapter VH. we find the descriptions of 

 individual kinds of rock treated under such 

 appropriate headings as: Mineral Composi- 

 tion; General Properties; Chemical Compo- 

 sition (the latter illustrated by a few well- 

 chosen analyses) ; Occurrence and Alteration. 

 Thus while the descriptions furnish one with 

 data useful for the recognition of the rock in. 

 the field or laboratory, they also furnish in a 

 most satisfactory manner what might be 

 termed the Natural History of the rock. 



With so excellent a manual available as a 

 basis for petrological study it is to be hoped 

 that it may be substituted for the dry and gen- 

 erally uninteresting lecturing so common in 

 petrological laboratories, as well as for the too 

 numerous laboratory guides and notes of in- 

 ferior quality, and thus promote a general im- 

 provement in petrological teaching. 



In conclusion it may be said that the book 

 is conspicuous by reason of its typographical 

 excellence and for the superior quality of the 

 illustrations which admirably illustrate the 

 text. Charles H. Warren 



Massachusetts Institute of Technology, 

 Boston, Mass. 



Die Pendulationstheorie. Von Dr. Heinrich 

 SiMROTH, Professor an der Universitat 

 Leipzig. Octavo, S. 564, Karten 2Y. Leip- 

 zig, Konrad Grethlein's Verlag, 1907. Pr. 

 brosch, M. 12. 



Theories of polar pendulation, based on 

 geological or geographical considerations, 

 have been proposed independently in the past 

 few years by two investigators, one^ an engi- 

 neer, another^ a geologist. To Professor 

 Simroth alone among biologists, however, does 

 the idea seem to have appealed as worthy of 

 attention for its possible value in explaining 

 the facts of zoogeography. 



' Paul Reibisch, " Ein Gestaltungsprinzip der 

 Erde," 27 Jahresher. Ver. f. Erdk. zu Dresden, 

 1901, S. 105-124. II., ihid., 1905, S. 39-53, 2 

 Karten. 



^ D. Kreichgaur, " Die Aquatorfrage in der 

 Geologic," Steyl, 1902. 



