Petrological Abstracts and Reviews 



Edited by ALBERT JOHANNSEN 



McLiNTOCK, W. F. P. Guide to the Collection of Gemstones in the 

 Museum of Practical Geology. London, 191 2. Pp.92. 

 This pamphlet is much more than a guidebook, the first thirty-four 

 pages being devoted to a general discussion of the properties of gem- 

 stones, the manner of cutting, etc. The remaining pages are devoted to 

 descriptions of the different gems found in the collection, information 

 being given in regard to crystal form, occurrence, methods of cutting, 

 coloring, etc. American museums might well follow the plan of those of 

 Scotland and England in issuing these useful guides. A. J. 



Mead, W. J. "Some Geological Short-Cuts," Econ. Geol., VII 

 (191 2), 136-44. 

 The author gives some ingenious methods for shortening the labor of 

 various computations. The first of these is a graphical method for the 

 determination of the composition of a rock from its mineral constituents. 

 It consists in reading directly from diagrams the component oxides of 

 the minerals. Unfortunately for the majority of petrographers, the 

 author reproduces only one table from a set of fifty-four of rock-forming 

 and thirty-seven of ore minerals, prepared in blue print form for his 

 students at the University of Wisconsin. For the same purpose, but in 

 much more convenient form, is the author's "geologist's slide rule." 

 This consists of a circular diagram, based on logarithmic principles. 

 The percentages are read by means of an inner rotating disk and a 

 celluloid arm. In the present slide rule, thirty-six common rock-forming 

 minerals are included, and the author has in preparation a similar one 

 for thirty-eight of the principal ore minerals. A third short-cut is the 

 "straight-line method" by which direct comparison may be made, 

 without recalculation, of analyses of fresh and altered rocks to show the 

 change in composition due to alteration. A. J. 



Mennell, F. p. a Manual of Petrology. Chapman & Hall, 

 London, 1913. Pp. 256, figs. 124. 'js. 6d. net. 

 This work, originally planned as the third edition of Mennell's 

 Introduction to Petrology, was so much changed during revision, that a 



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