428 Reports and Proceedings — British Association. 



salts, the eidectic, which crystallises at the lowest possible 

 temperature, and is the only mixture which has exactly the same 

 composition as the liquid from which it solidifies. Teall made the 

 illuminating suggestion that micropegmatite is an eutectic consisting 

 of quartz and felspar, and represents in certain rocks the final 

 mother liquor from which the other minerals have crystallised out. 

 Eutectics in metallic alloys have been much studied during recent 

 years ; in the Address of 1901 Teall was able to strengthen his case 

 by showing that sj^herulitic and micropegmatitic structures found in 

 obsidian and other acid rocks are pai'alleled by similar structures 

 developed in eutectic alloys, according as they have been rapidly or 

 elowly cooled. 



In the following year appeared a theoretical paper by Meyerhoffer 

 concerning the ideal case of a molten mixture of two substances, 

 a and b, which do not suffer double decomposition, nor form a double 

 salt, nor an isomorphous mixture. • 



Let a diagram be constructed, with temperatures as ordinates and 

 composition of the magma as abscissa3, giving by a curve the nature 

 of the magma which is in equilibrium with either solid a or solid h. 

 The curve has the form of a V ; one arm represents the temperature 

 and constitution of the liquid which can be in equilibrium with a, 

 and the other that of the liquid which can be in equilibrium with b ; 

 and the lowest point corresponds to the eutectic, which is in contact 

 with both. 



Let a point above the curve represent the temperature and 

 constitution of the liquid magma containing excess of 6 ; as the 

 magma cools this point descends to the b branch and travels along 

 it while b is crystallising out, until the eutectic point is reached, 

 when a and 6 both crystallise out together at a temperature below 

 the melting-point of either. The oi'der of crystallisation is therefore 

 determined solely by the composition of the magma as compared 

 with that of the eutectic. If, however*, the liquid be cooled slowly, 

 crystallisation may be postponed until it has become supersaturated 

 with regard to one constituent or the other, or both ; a state of afiairs 

 represented by a prolongation of the arms of the V below its lowest 

 point, and then the order of the crystallisation may be inverted. 



In a rock-magma there are of course many other factors to be 

 taken into account as determining the order in Avhich the minerals 

 separate ; for example, the formation of both double salts and 

 isomorphous mixtures, the possible production of unstable solid 

 compounds which may become converted into stable compounds or 

 may be redissolved soon after they have come into existence ; and 

 also the relative velocities of crystallisation, changes of temperature 

 and pressure, action of steam, etc. ; but the principle laid down by 

 Meyerhoffer must be that which controls the process. 



It might be objected that on this hypothesis the consolidation 

 of every rock-mass ought to terminate with an eutectic mixture, 

 whereas this appears to be by no means the case ; in fact, it is only 

 among some acid rocks that structures much resembling the eutectic 

 mixtures of alloys are to be found. On the other hand, if the 



