Reports and Proceedings^British Association. 519 



near Gar el Gehannem (pi. xv) illustrates the difficulties wbioli often 

 impede the traveller in crossing such desert tracks, some of these 

 dunes being practically impassable by caravans for miles. 



Of the curious blocks of sandstone, pierced by numerous borings, 

 we shall have something more to say on another occasion. They 

 appear to be the exact replica of specimens brought home from Lake 

 Tanganyika by Mr. J. E. S. Moore. 



The maps and sections are most excellent. We congratulate the 

 author and Captain H. G. Lyons, F.G.S., the Director General of the 

 Surveys Department, on this most admirably executed memoir. 



lEaSI^OIEaTS .A-IsTZD IFieOGZEEHDHsTGrS. 



British Association for the Advancement of Science. 

 Cape Town, South Africa, August 16th, 1905. 



Address to the Geological Section (C) by Professor H. A. Miers, M.A., 

 D.Sc, F.R.S., President of the Section. 



{Concluded from the October Number, p. 478.) 

 Supersaturated Solutions. 



I DO not myself see how we can do otherwise than apply to the 

 study of rock-magmas all that can be learnt from physical chemists 

 concerning the behaviour of solutions, for though we cannot attain 

 in laboratory experiments the high temperatures and great pressures 

 at which rocks may have crystallised, there is no reason to believe 

 that these introduce more than a difference of degree. The principles 

 of equilibrium between the various crystallising components probably 

 remain the same, whatever may be the temperatures and pressures 

 at which they have solidified. 



It must at the same time be confessed that most of the experiments 

 upon which the modern theory of solutions has been built up have 

 been conducted, upon dilute solutions, whereas the problems of 

 crystalline growth are concerned, not with dilute nor even with 

 saturated solutions, but only with solutions which are supersaturated. 

 There is some force in the objection of Doelter that the results of 

 such experiments may not be directly applicable to crystallising slags. 



For example, as I have already mentioned, doubt has been 

 expressed in the case of silicate magmas, whether the substances in 

 solution are the minerals about to crystallise or only their constituents ; 

 whether viscosity and supersaturation may not invert the theoretical 

 order of their appearance ; whether we are to take into account 

 possible dissociation of the molecules or not ; whether the presence 

 of a common ion in these minerals is a factor which determines 

 their mutual solubility. In fact, very little is known about the 

 actual condition of the materials in a strong solution, altliough I do 

 not know that there is any evidence available which forbids us to 

 regard a solution about to crystallise as a mixture of liquids one of 

 which is about to pass into the solid state. 



But if little is known about the nature of strong and supersaturated 



