Prof. S. J. Shand — Saturation in Petrograi:)hy. 491 



pitchstone-porphyrit's from Arran to add to the great weight of 

 observations in favour of my view. We in this country are greatly 

 handicapped in the matter of literature, and the paper in question 

 had escaped my notice. In regard to the quartz-basalts of California 

 and New Mexico, Lacroix'has opposed the views of Iddings and 

 Diller, contending, not only on general grounds but also as the result 

 of his own examination of the rocks in question, that the quartz of 

 these basalts is really of foreign origin; he compares the rocks with 

 lavas of Central France in which xenocrysts of quartz and felspar are 

 just as evenlj' distributed, and yet are clearly of external origin. 



Falling short of field evidence, Mr. Scott advances some theoretical 

 considerations against my view. One of these is that the reaction 



olivine + silica — >• enstatite 



may be appreciably reversible under magmatic conditions. If so, 

 where is the evidence of it? It cannot be mere chance which decrees 

 that olivine and enstatite or hyperstheiie shall commonly occur 

 together, that hyperstliene and quartz shall be associated still more 

 frequently, but that quartz and olivine shall never coexist except 

 in circumstances which suggest that one is of extraneous origin. 

 In some cases an olivine-bearing magma has been frozen in the 

 very act of absorbing quartz and developing hyperstheue or another 

 pyroxene ; numerous instances of this will be found in Lacroix' 

 studies of the reactions between lavas and their inclusions." Mr. Scott 

 has himself explained tlie appearance of hypersthene in a rock at 

 Garabal Hill by reaction between olivine and quartz.- Negative 

 evidence as to the incompatibility of these two minerals is extra- 

 ordinarily abundant, and he will find some of it at his hand 

 in the intrusions of Kilsyth-Croy, which, according to Tyrrell,* 

 contain hypersthene where quartz is present, and olivine only where 

 quartz is absent. Even laboratory work points in the same direction. 

 l)aubree ^ found that when he melted olivine in siliceous crucibles, 

 the melt attacked the crucible with great energy and formed 

 pyroxenes. He concluded from this that the rarity of peridotites 

 in nature is due to the fact that the peridotitic magma tends to 

 dissolve siliceous rocks and so to change its character. 



Lastly, Mr. Scott quotes against my view the work of Anderson 

 and Bowen * — which comes hot from the printing-press — on the 

 binary system MgO-SiOg. Here at least is evidence worthy of 

 consideration. When a melt of the composition Mg Si Oo was cooled 

 down from a veiy high temperature, forsterite was found to crystallize 

 out at about 1,625°, and was followed later by a mixture of Mg Si O3 

 and Si Oo in the forms of clino-enstatite and cristobalite (probably). 

 But Anderson and Bowen go on to show that forsterite is only 

 produced above 1,557°; when crystallization takes place at any 

 temperature between 1,200° and 1,557°, only clino-enstatite is formed. 

 The latter temperature is in f;ict the point at which clino-enstatite 



^ Les Enclaves des Roches Volcayiiques, p. 43. 



- Geol. Mag., November, 1913. ^ Ibid., July, 1909, p. 299. 



■* Cited by Lacroix, loc. cit., p. 498. 



^ Amer. Journ. Sci., xxxvii, and Zeit. Anorg. Chem., Ixxxvii, 1914. 



