322 Alexander Scoit — Saturation of Minerals. 



relations of these two groups, the evidence both of petrography 

 and of synthetic work ^ is -in favour of pyroxene being the high 

 temperature form and hornblende the low temperature one. In the 

 diorites and gabbros, hornblende often appears as a parallel growth 

 on pyroxene, and many cases have been described of the former 

 mineral containing cores of the latter.* Similarly augite is often 

 found pseudomorphed by hornblende. It has been argued against this 

 view that the hornblende, and the pyroxene from which it forms, 

 have different compositions, particularly vphere the alteration has 

 been aided by pressure, as in the formation of hornblende schist by 

 the alteration of a dolerite.^ This difference, however, is probably 

 due to the fact that these minerals are capable of combining many 

 molecules in solid solution, and that this capability varies with the 

 conditions of crystallization. Thus, at Garabal Hill,* the margin 

 of an intrusion is a hornblende diorite with about 30 per cent of 

 hornblende, and the centre, a few feet distant, is a hornblendite 

 with 85 per cent of hornblende, while the composition of the two 

 rocks is approximately the same. The cases of alteration of horn- 

 blende into pyroxene would seem to be explained by reheating due 

 to later intrusions,* or to some similar cause. 



Another change which may take place in the solid state is the 

 breaking up of a solid solution on cooling. Vogt^ describes the 

 separation of augite from solid solution in enstatite and also ascribes 

 this origin to some perthitic growths.'' 



So far, only the effect of temperature on the formation of stable 

 minerals has been considered. The conditions of pressure, however, 

 also exert a considerable influence, particularly in many plutonic 

 rocks. When an igneous rock contains minerals which are generally 

 of metamorphic origin, it is assumed that these have formed under 

 pressure. For example, garnet is a typical product of metamorphism, 

 and when it does occur in a plutonic rock it has probably crystallized 

 under high pressure. In the case of the Charnockite Series, the 

 garnet seems to have originated in this way, as there is other evidence 

 of the rocks having undergone dynamic action,^ while the eclogites 

 in many cases show similar effects.^ Fermor ^° postulates for the 

 formation of kodurite, an infra-plutonic zone of garnet-bearing rock. 



The evidence of synthetic work is likewise in favour of garnet 

 forming only under particular conditions of pressure and probably 



^ Allen, Wright, & Clement, Amer. Journ. Sci., ser. iv, xxii, pp. 385-438, 

 1906; Allen & White, ibid., ser. iv, xxvii, pp. 1-47, 1909; Doelter, Neues 

 Jahrbuch, 1897, pt. i, pp. 1-26. 



2 Cf., for example, Allporfc, Q.J.G.S., xxxv, pp. 637-42, 1879; Wyllie and 

 Scott, Geol. Mag., Dec. V, Vol. X, pp. 499-508, 536-45, 1913. 



2 Teall, Q.J.G.S., xli, pp. 133-44, 1885; see also Harrington, Eec. of 

 Progress. Geol. Surv. Canada, 1877-8, p. 21 G. 



'' Wyllie & Scott, loc. cit. 



'' Cf. Lacroix, Miniralogie de la France, i, pp. 668-9, 1895. 



« Q.J.G.S., Ixv, pp. 100-1, 1909. 



"' Tscher. Min. u. Pet. Mitt., xxiv, pp. 537-42, 1905. 



8 Holland, Mem. Geol. Surv. India, xxviii, pp. 117-249, 1898. 



** Cf. Hezner, Tscher. Min. u. Pet. Mitt., xxii, pp. 437-505, 1903, for origin 

 of eclogites. 

 " Bee. Geol. Surv. India, xlii, pp. 208-30, 1912 ; xliii, pp. 41-7, 1913. 



