538 B. K. N. Wyllie <& A. Scott— Plutonics of Garabal Hill. 



GeNEEAL PKTROLOGr. 



Excluding for the moment the tonalite, and also the possible 

 relations between the diorites and the ultrabasic rocks, we propose 

 to consider shortly the variations exliibited by each of these two 

 groups. Althougli chemical analyses have not been made of all 

 the rocks concerned, it seems most probable that there is no great 

 variation in chemical composition in the ultrabasic series, save where 

 impregnation by the latter acid intrusives has occurred. The basic 

 end-member is a pure olivine rock altered to serpentine, and with 

 a composition approximating to that of olivine. The otlier end- 

 member is the pyroxenite. The mineralogical variation could 

 therefore be shown diagrammatically by two curves, one representing 

 the increase of pyx-oxene — including hornblende — from zero to 

 approximately 100 per cent, while the other would represent the 

 decrease of olivine from 100 per cent to nearly zero. The chemical 

 differences ai'e so small, therefore, that it seems to us that local 

 variations in the conditions are sufficient to account for them. That 

 the intrusion did not take place all at one time is shown, by tlie 

 presence of the veins of porphyritic rock, the end-product of the 

 consolidation of the magma. Hence, in a magma of such size, 

 consolidating over a considerable period of time, the conditions could 

 not remain uniform. Local variations in temperature and pressure 

 would involve local partial consolidation, and therefore areas of lower 

 potential, with the result that a potential gradient would be set up. 

 This would be followed bj- intermagmatic chemical reactions, involving 

 the formation of other molecules and resulting in local excesses of 

 ortho- or metasilicate molecules, with an ultimate consolidation as 

 olivine or pyroxene, as the case may be. 



A similar explanation would account for the variations of the 

 diorites. Here again we have a comparative uniformity in chemical 

 composition, the chief variation being in the mineralogical composition, 

 in the substitution of pyroxene by hornblende and of hornblende by 

 felspar and biotite. iSupersaturation of the magma with respect to 

 the ferromagnesian molecules would not occur everywhere at the 

 same temperature, with the result that sometimes the high temperature 

 modification, pyroxene, would form, sometimes the modification stable 

 at lower temperatures, hornblende. The amount of water dissolved 

 in the magma would also exert some effect, as the presence of water 

 favours the formation of hornblende, in addition to reducing the 

 viscosity. Local richness in potash would affect the production of 

 biotite, Avhile pressure M'ould be an important factor owing to the 

 probably high molecular volume of mica. A magma from which 

 under certain conditions pyroxene would crystallize, would, under 

 other conditions, give hornblende or biotite.^ Hence it seems to us 

 that there exists no necessity to postulate any differentiation beyond 

 what would ensue from a locally heterogeneous magma. 



We will now proceed to consider the Garabal Burn series in greater 

 detail. The texture varies greatly, as in places chilled margins 



^ Cf. Doelter, Neues Jahrb. fiir Min., pt. ii, p. 178 et seq., 1888 ; pt. i, p. 1 

 et seq., 1897. 



