Reviews — A. W. Rogers — Geology of Cape Colony. 135 



operative, and how far it may affect the mutual relations of other 

 minerals — olivine, enstatite, augite, and biotite all having this same 

 ion in common. Any quantitative estimate, depending on the degree 

 of electrolytic dissociation of the several constituents, will not easily 

 be arrived at. The author will not allow that there may be only 

 a limited mutual solubility between spinel, apatite, etc., and the 

 silicates, though he finds this kind of relation between sulphides 

 and silicates. 



We can notice here only the main features of what is virtually 

 the outline of a consistent theory of the crystallization of igneous 

 rocks. Many other points are touched, sometimes comprehensively, 

 sometimes sketchily, but always in a suggestive manner. They 

 include, for example, the important consequences of supersaturation 

 in the magma ; the modifying influence of water and other so-called 

 ' mineralisers,' and generally the question how far results derived 

 from the study of slags are applicable to plutonic and other natural 

 rocks; the application of the solution-theory to magmatic differ- 

 entiation ; and other petrological problems. The porphyritic structure, 

 for instance, becomes from the author's point of view a general 

 character, not necessarily implying a discontinuous change of 

 physical conditions between two distinct ' periods of consolidation.' 

 The porphyritic elements may represent the excess of one or more 

 constituents separated out at an early stage, while the ground-mass 

 tends often to an approximately eutectic composition. This hypo- 

 thesis again is examined by the test of Lagorio's analyses. 



We may hope that the outline presented by Professor Vogt will 

 be filled in by the researches of himself and other petrologists. If 

 modifications on some minor points should be found necessary, as 

 well as additions, this will not lessen the debt which students of 

 igneous rooks owe to the author of the valuable memoir before us. 

 A. H. 



IV. — An Introduction to the G-eology of Cape Colony. By 

 A. W. EoGBRS, M.A., F.Gr.S., Director of the Geological Survey 

 of Cape Colony. 8vo ; pp. xviii and 463, with a folding map 

 and 21 illustrations. (London : Longmans, Green, & Co., 1905. 

 Price 9s. net.) 



THE student of South African geology, after much time spent in 

 reading numerous Government reports, treatises, and articles 

 scattered through different magazines, at home and abroad, usually 

 gives up the task in hopeless bewilderment, and ends with the 

 conviction that " nothing is but what is not." He finally develops 

 that fatal African disease known as the sleeping sickness, when all 

 further interest in the subject naturally ceases. At what stage, if 

 any, geologists in this country have arrived is a matter of conjecture 

 and is hardly likely to be known. If such an unfortunate mortal 

 exists good news awaits him. After reading the book by Mr. Rogers 

 he will awaken like a giant refreshed. 



The present volume is the first of a series designed by Dr. Muir, 

 the Superintendent General of Education for the Colony of the Cape 



