'Geological Society. 69 



trical discussion of heinihedrisin and tetratohedrism. These some- 

 what difficult points are illustrated by shaded drawings, which give 

 the student a very clear idea of what heinihedrisin really means. 



The notations used are those of Miller and JNaumann, side by 

 side throughout — the figures bearing Miller's numbers on their 

 faces (except for the hexagonal system, in which Bravais's modifi- 

 cation is adopted), while Naumann's symbols are used in the 

 descriptive text. Explanations are also given of other systems, 

 such as those of Weiss and Schrauf. 



We notice as minor deficiencies in the geometrical portion of the 

 work, first, that it might have been mentioned a little more distinctly 

 that the unclosed forms, which occur in all except the cubic system, 

 can only occur in combination ; secondly, that in the account of 

 twin crystals (p. 167) the distinction between the twin-plane and 

 the composition-plane is not very fully explained, and is not illus- 

 trated specifically ; although a figure of the Carlsbad twin, in which 

 these two planes do not coincide, is given elsewhere (p. 181). 



These are, perhaps, not very important points : the first will 

 speedily force itself on the attentive student ; and as regards the 

 second, it may be said that no one has a right to assume the iden- 

 tity of two things which are arrived at by different considerations 

 without proving them to be identical. 



The optical properties of crystals are considered at somewhat 

 greater length than is usual in elementary treatises ; and compre- 

 hensive tables are furnished of the optical constants of the trans- 

 parent minerals. 



In the chemical portion of the work, the author has adopted the 

 classification followed by Rammelsberg in the second edition of his 

 Handbuch cler Mineral. Chemie, which is now regarded as the stan- 

 dard authority upon the chemistry of minerals. 



The present volume, which completes the systematic part, is 

 to be followed by another upon descriptive Mineralogy. If the 

 forthcoming volume be prepared with the same ability and pains- 

 taking care which is exhibited in that now before us, the two will 

 constitute a reliable elementary textbook which cannot fail to be 

 of the greatest value to students of mineralogy. 



VIII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from vol. xi. p. 541.] 



April 6, 1881.— J. W. Hulke, Esq., F.E.S., Vice-President, 

 in the Chair. 

 npHE following communications were read : — 

 -*- 1. " The Microscopic Characters of the Vitreous Eocks of Mon- 

 tana, U.S." By E. Eutley, Esq., E.G.S. ; with an Appendix by 

 James Eccles, Esq., E.G.S. 



The specimens described were collected by Mr. J. Eccles, P.G.S. 

 They consist of various obsidians and rhyolites, some of them porphy- 



