Reviews — Fletcher'' s Study of Rocks. 513 



since the date of the G-erman edition, and acoount for the large 

 amount of additional matter which appears in the English trans- 

 lation. 



Osmotic pressure, diffusion, the nature of double salts and 

 isomorphous mixtures, the equilibrium between different substances 

 in a mixed solution — all these are subjects of vital importance to 

 the chemist, and of no less interest to the geologist who occupies 

 himself with the study of igneous magmas. Important above all 

 is the experimental work of Roozeboom upon the last-mentioned 

 problem. About a third of the book is devoted to a sketch of 

 isomorphism, under which these points are ably treated. There is, 

 of course, mixed with established facts much that is hypothetical, 

 and little attempt is made by Dr. Fock to sift the one from the 

 other ; but he has, at any rate, given a truthful abstract of work 

 which was previously only to be found dispersed through many 

 books and journals. 



Mr. Pope has made an excellent translation, and has done good 

 work in bringing the subject to the notice of English readers. 

 The result of his collaboration with Dr. Fock has been to supply 

 a thoroughly attractive and readable survey of all that is known at 

 the present date about chemical crystallography or crystallographic 



chemistry. 



II. — An Introduction to the Study of Eocks. By L. Fletcher, 

 F.R.S., etc., Mineral Department, British Museum (Natural 

 History). Printed by Order of the Trustees: London. 8vo, 

 pp. 118. Price 6(Z. 



THIS Handbook may well be considered alongside of the Guide 

 to the Collections of Rocks and Fossils belonging to the 

 Geological Survey of Ireland.^ Both in a measure show the diffi- 

 culties of grouping information which relates to diverse specimens 

 arranged in cases ; but the methods adopted in the two works are 

 in many respects widely different. In the Survey Guide, the age, 

 method of formation, and life-history of the rocks are dominant 

 features; in the British Museum Guide, greater prominence is given 

 to their mineral structure and characters. 



Commencing with definitions of different kinds of rock and of 

 large mineral masses, the author proceeds to describe their varieties 

 of structure and composition, and the methods of crystalline growth. 

 In reference to characters he uses the term petrical for those to be 

 observed in large rock-masses in the field ; and lithical for the 

 characters manifested by fragments of rocks, such as are preserved 

 in a museum. Then follow concise and excellent explanations of 

 many terms commonly used to describe these various characters. 



The variations in rock-masses and the similarities between rocks 

 of different ages are duly noted ; and it is pointed out that the same 

 mass may at one age belong to one kind, at another age to a different 

 kind of rock, as a result of metamorphism. Serpentine is cited as 

 an example. 



The author then expresses his determination " that rocks should 

 1 Infra, p. 516. 



DECADE IV. VOL. II. NO. XI. 33 



