1893. SOME NEW BOOKS. 465 



congratulate the author on the completion of his task — one which, 

 considering its comprehensive character, must indeed have proved 

 an immense labour ; but which, for this very reason, is a signal 

 service to Science. 



Tables for the Determination of the Rock-forming Minerals. Compiled 

 by Professor F. Loewinson-Lessing, translated from the Russian by J. W. 

 Gregory, B.Sc, F.G.S., with a chapter on the Petrological Microscope by 

 Professor Grenville A. J. Cole. M.R.I.A.. F.G.S. London : Macmillan & Co., 

 1893. Price 4s. 6d. net. 



Many investigators who have not had the chance of proper training 

 in a petrographical laboratory, and have had to gain their acquaintance 

 with the methods by books, must have felt their lot a very hard one. 

 Even when they have become fairly well practised in the branch of 

 the microscopic mineralogy of rocks, the determinations in many 

 cases are long and tedious, as classified diagnostic tables were still 

 wanting. So far we had only the systematic description of minerals 

 as they occurred in rocks, but they were classified according to their 

 chemistry, and not by their microscopic characters. The above work 

 admirably fills that gap, and now, after a few lessons in microscopical 

 tactics, a student with a fair knowledge of mineralogy can start as a 

 rock describer. 



The original work appeared in Russian, a language to all intents 

 and purposes beyond the knowledge of European and American 

 scientists, and Dr. J. W. Gregory has made all petrographers his 

 debtors for translating Professor Lce\vinson-Lessing's book. Dr. 

 Gregory had intended to add a chapter on the petrological microscope, 

 but one of his multifarious duties called him to African exploration, 

 so that his partially-finished chapter was placed in the able hands of 

 Professor Grenville A. J. Cole, who has acted as the contributor of it 

 to this joint work. 



In the introduction we have a general outline of the methods to 

 be employed. In the next chapter the main requisites of a petro- 

 graphical microscope and their uses are described. In part II. of the 

 book we have the tables. Minerals are divided for investigation into 

 opaque, semi-opaque, and transparent, and under each the optical, 

 chemical, and other tests are given. The transparent are divided 

 into colourless and coloured. These groups are subdivided again, 

 according to whether the mineral shows crystal outlines, crystalline 

 grains, or grains in crystalline rocks. Table III. gives a classification 

 of other morphological characters, such as tabular crystals, needles, 

 broad tablets, flakes, filaments, or aggregates, while the presence of 

 colour is again used to group the minerals. In table IV. the deter- 

 mination of the crystalline system under the microscope is treated of. 

 This is followed in table V. by a classification of the rock-forming 

 minerals according to their crystalline system. The book terminates 

 by a table of minerals according to their optical signs. 



Only one or two questionable statements attracted our attention, 

 one being, " It is convenient to leave leucite in the prismatic system, 

 which it resembles in form ; it appears, however, to be really a 

 monoclinic mineral which has acquired prismatic form by compound 

 mimetic twinning." Most of us hoped that the interminable literature 

 and ingenious contrivances of transcendental crystallographers to 

 explain the optical anomalies of leucite were once for all finished, 

 and that simple temperature strain in an isometric mineral had been 

 accepted as the cause. 



211 



