Geology and Mineralogy. 79 



forming an angle of 72° with the base are frequently seen between 

 crossed nicols; the extinction is at times parallel to the edges of 

 the base, but generally with a slight inclination (maximum 15°). 

 Pleochroism strong ; c == greenish yellow, b — plum-blue, a = 

 olive-green. 



The layers of schist with ottrelite often alternate with layers of 

 the thickness of the finger of schist with magnetite and of normal 

 schist free from mineral inclusions. 



7. Estimation of the Optical Angle: A correction by Alfred 

 C. Lane. (Communicated). — In my article on the estimation of 

 the optical angle in this Journal,* I notice an error, which I 

 knew enough to have avoided, as the whole subject was long ago 

 thoroughly treated in the " Mineralogie Micrographique " of 

 Fouque and Levy. The maximum extinction-angle in the pris- 

 matic zone of common hornblendes is not the clinopinacoidal 

 section. This the above writers explained theoretically and I have 

 experimentally verified the fact. The extinction-angle increases 

 slightly and then decreases, and even on the cleavage faces the 

 extinction-angle is but little less than in the clinopinacoidal sec- 

 tion. 



8. Systematic Mineralogy based upon a Natural Classification, 

 with a general introduction, by Thomas Sterry Hunt. 391 pp. 

 8vo. New York, 1891. (The Scientific Publishing Company.) — 

 This volume is a development of Dr. Hunt's views on mineral 

 classification, as in part before brought out, more particularly in 

 the volume entitled " Mineral Physiology and Physiography." 

 The work covers, however, a rather wide range. It begins with a 

 historical summary of the different systems since the time of Wer- 

 ner; then follows an explanation of the principles of chemistry 

 and chemical notation as related to the subject in hand, and 

 further the special subject upon which the author has laid most 

 stress : the relation of specific gravity, hardness and solubility to 

 mineral condensation, finally a chapter on crystallization. This 

 occupies about one-half the work, after which follow the " new 

 mineralogical classification " and synopsis of mineral species ; in 

 the latter the successive species recognized by the author, ar- 

 ranged by genera in tabular form with summary of chief charac- 

 ters, are provided with dual Latin names after the methods of 

 Natural History and further each "genus" is briefly character- 

 ized. For an understanding of the methods employed and 

 detailed results reached, the student must be referred to the 

 original volume. How far the new system and nomenclature are 

 permanent contributions to science, the future will show. 



9. The Phosphates of America. When and how they occur; 

 how they are mined, and what they cost. With practical trea- 

 tises on the manufacture of sulphuric acid, acid phosphate, phos- 

 phoric acid and concentrated superphosphates, etc.; by Francis 

 Wtatt. Second Edition. 187 pp. 8vo. New York, 1891. (The 

 Scientific Publishing Company.) — The phosphate industry in this 



*This Journal, vol. xxxis, 1890, p. 56, line 1. 



