340 SCIENCE PROGRESS 



Principles of Chemical Geology. A Review of the Application of the Equilibrium 

 Theory to Geological Problems, by James Vincent Elsden, D.Sc, F.G.S. 

 [Pp. viii + 222.] (London : Whittaker & Co., 1910. Price 5^. net.) 



It is only by slow degrees that geology is establishing its claim to be regarded as 

 an exact science. As long ago as 1857 Sorby attempted to obtain quantitative 

 determinations of the conditions under which igneous rocks were consolidated, 

 and his last communication to the Geological Society in 1908, the year in which he 

 died, was concerned with the application of similar methods to sedimentary strata. 

 In Sweden the first steps to a definite geological chronology in years has now at 

 last been taken ; but it is only recently that it has been generally recognised that 

 the rocks of the earth's crust contain as a rule in their physical characters data which 

 will serve as the basis of calculations with regard to the conditions that prevailed 

 when they were formed. 



In order, however, that these records should be satisfactorily interpreted, a com- 

 petent knowledge of the principles of chemical physics and of their application to 

 magmas, minerals and rock masses must be obtained. This at the present moment 

 we do not possess. Much of the theory is still in dispute, and the determination 

 of physical constants under conditions of high temperature and pressure is still 

 very incomplete, though excellent work in this direction is now being carried out by 

 Dr. Day and his collaborators at Washington. 



A considerable body of literature has, however, already come into existence, 

 much of it scattered through physical, chemical, mineralogical and petrological 

 periodicals of Europe and America, and we are under very considerable obliga- 

 tion to Dr. Elsden for providing us with a handy digest of the researches that have 

 already been carried out in this difficult field of research, and some discussion as 

 to the conclusions to be drawn from them. In the present imperfect state of our 

 knowledge he has acted wisely in endeavouring to make his book as comprehensive 

 as possible, although in the result his exposition of important principles is some- 

 times all too brief The full references which are furnished throughout will, however, 

 always enable the worker to study the original authority at first hand if he wishes 

 to do so. The time is not yet ripe for the preparation of a simple introductory text- 

 book on the subject. 



Among the numerous topics with which the author deals are crystallisation 

 from fusion or solution ; diffusion ; viscosity ; surface tension in its manifold rela- 

 tions ; vapour pressure especially in connexion with hydration ; and the transfor- 

 mation of polymorphous forms into one another. Everywhere change is shown to 

 proceed from a system of greater energy to one with less till a condition of stable 

 equilibrium is reached, though millions of years may pass before the process even 

 approaches completion. 



In his anxiety to hold the balance true between different authorities, the author 

 is sometimes unduly cautious. He declares himself, for instance, uncertain whether 

 the presence of volatile fluxes would counteract the natural viscosity of acid igneous 

 magmas ; though there are, I imagine, few facts so well established as the extreme 

 fluidity of some of the niore siliceous intrusions connected with granite masses, and 

 there can be no doubt that this must be attributed mainly to the presence of water. 

 The suggestion, on the other hand, that greisen represents the mother liquor of a 

 granite magma is scarcely likely to be verified. The more usual view that it is 

 the result of the action of residual volatile constituents on a portion which has 

 already crystallised in whole or in part would seem to be much more probable. 

 Again, the supposition that the separation of a marginal facies would be in inverse 



