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THURSDAY, AUGUST i, i{ 



MICROSCOPICAL MINERALOGY. 



Petrographical Tables : an Aid to the Microscopical 

 Determination of Rock-forming Minerals. By Prof. 

 H. Rosenbusch. Translated and Edited (with the 

 Author's permission) by Dr. F. H. Hatch, of H.M. 

 Geological Survey. (London : Swan Sonnenschein 

 and Co., 1889.) 



DR. HATCH has rendered a great service to English- 

 speaking students by the preparation of this careful 

 translation of Prof. Rosenbusch's very admirable " Hiilfs- 

 tabellen zur mikroskopischen Mineralbestimmung in Ge- 

 steinen," which appeared a few months ago. These tables 

 mayindeed be regarded as an index or summary to the same 

 author's invaluable " Mikroskopische Physiographic der 

 petrographisch wichtigen Mineralien," the English trans- 

 lation of the second edition of which, by Mr. J. P. Iddings, 

 was reviewed not long a.;?o in the pages of Nature. The 

 two works together supply a want that has long been felt 

 by English students — namely, a complete summary of all 

 that has been done in the way of making the optical and 

 other characters of minerals available as means of recog- 

 nizing them when seen in thin sections of rocks under 

 the microscope. 



Of the admirably lucid and exhaustive manner in which 

 Prof Rosenbusch, in these works, has compared, ar- 

 ranged, and not unfrequently verified the results accumu- 

 lated by Zirkel, Tschermak, Von Lausaulx, Fouqud, 

 Michel Levy, and a host of other observers, who, during 

 the last thirty years, have followed up the suggestive 

 work of Mr. Sorby, the original founder of the science 

 of microscopic petrology, it is not necessary to speak in 

 this place. Geologists and mineralogists are alike placed 

 under a great debt of gratitude to one who — besides 

 making many original observations of great value him- 

 self — has so admirably systematized and correlated the 

 results obtained by a great number of other independent 

 workers ; nor can they forget that Prof. Rosenbusch is 

 the founder of the now famous Petrographical School of 

 Heidelberg, in which many of the foremost investigators 

 of this branch of science, now engaged in the study of 

 rocks on both sides of the Atlantic, have been so well 

 trained. 



The first attempt at the construction of a series of 

 tables, on something like the same lines as those in the 

 work before us, was made as long ago as the year 1876, 

 by Dr. C. Doelter, of Vienna, a pupil of Prof. Tschermak. 

 It was only three years before this that the earliest en- 

 deavours to systematize the results arrived at by the 

 study of rocks, with the aid of transparent sections under 

 the microscope, had been given to the world in Zirkel's 

 " Die mikroskopische Beschafifenheit der Mineralien und 

 Gesteine," and in Rosenbusch's "Mikroskopische Physio- 

 graphie der petrographisch wichtigen Mineralien": the 

 former dealing more particularly with various charac- 

 teristic peculiarities of the different rock-forming minerals, 

 as a means by which they may be identified in thin 

 sections ; the latter aiming at the utilization of the im- 

 portant results arrived at by Des Cloizeaux and his pupils, 

 in their important determinations of the " optical con- 



VOL. XL.— No. 103 1. 



stants " of minerals, as a basis for the exact diagnosis of 

 the constitution of rocks, under the diflScult conditions 

 presented by very thin sections which intersect the 

 crystals in varying and unknown directions. Doelter's 

 tables, appearing after these systematic works, constituted 

 a very valuable addition to the scanty petrographical 

 literature of that day. In this work of Doelter, " Die 

 Bestimmung der petrographisch wichtigeren Mineralien 

 durchdas Mikroskop; eineAnleitung zur mikroskopischen 

 Gesteins-Analyse," an attempt is made to construct a 

 mineralogical key, similar to the "keys" published with 

 certain floras, and so familiar to all students of systematic 

 botany. Every mineral observed under the microscope 

 was to be submitted to certain optical tests, and then 

 relegated to a particular class ; and by the further applica- 

 tions of the similar tests new points of distinction were 

 to be detected, till at last the species or variety had been 

 correctly identified. The process recommended to be 

 pursued was, in fact, exactly similar to that which is 

 usually followed by chemical students in the work of 

 qualitative analysis. 



In 1885, Dr. Eugen Hussak, of Gratz, a pupil of Dr. 

 Doelter, published his " Anleitung zum Bestimmen der 

 Gesteinbildenden Mineralien," in which the same method 

 of procedure is illustrated in greater detail, and with the 

 assistance of many new facts and methods that had been 

 discovered since the publication of Dr. Doelter's tables. 



In 1888, MM. A. Michel Levy and A. Lacroix issued 

 their very valuable work " Les Mindraux des Roches," in 

 which the characters of the rock-forming minerals are 

 also exhibited in a series of tables. In this treatise, the 

 authors — we think wisely — abandoned the idea of making 

 anything like analytical keys for the determination of the 

 rock-forming minerals, and arranged the various species 

 and varieties simply in alphabetical order. Just as few 

 practical botanists or chemists find it necessary to go 

 through the whole of the elaborate schemes of research 

 contained in the text-books, though these may be of much 

 educational value to a beginner, so no student of the 

 microscopic character of rocks is called upon to pursue 

 the exhaustive method of analysis illustrated in the tables 

 of Doelter and Hussak. As a rule, the result of a pre- 

 liminary examination of a mineral seen in a thin section 

 is to make it obvious that we are dealing with some one 

 out of two or three possible forms ; and what the observer 

 most needs for deciding between these, is a statement of 

 the characteristic and distinctive peculiarities of every 

 species, arranged in such a manner as to facilitate reference 

 to them. 



Prof. Rosenbusch, in his tables, combines to some 

 extent the methods of his predecessors. The main 

 grouping of the rock-forming minerals is into (l) singly- 

 refracting minerals ; (2) doubly-refracting uniaxial 

 minerals (there is an unfortunate misprint in Dr. Hatch's 

 translation of " biaxial " for " uniaxial" in table \\.b) ; and 

 (3) doubly-refracting biaxial minerals. In each of these 

 classes, the several species or varieties are arranged in 

 groups according to their general affinities, and irrespec- 

 tively of their system of crystallization. Dr. Hatch has 

 very wisely supplemented this arrangement by an alpha- 

 betical index, and has thus secured almost the same 

 facility of reference which is so conspicuous and valuable 

 a feature in the tables of Michel L^vy and Lacroix. 



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