﻿120 ME. R. H. EASTALL ON THE [May I9IO,. 



Some specimens are fairly even-grained and non-porphyritic r 

 whereas others show large phenocrysts of white felspar. The 

 minerals and structures seen are similar to those described in the 

 preceding cases. 



With regard to the classification of this rock, it is somewhat 

 difficult to assign its position in the absence of a trustworthy 

 analysis. The only published analysis appears to be that by Hughes 

 quoted by Clifton Ward, and reproduced by Harker for com- 

 parison, but this is obviously an impossible composition. There is- 

 not nearly enough alumina to form felspars and micas with the 

 alknlis present, to say nothing of the lime, magnesia, and ferrous- 

 iron, which also must exist in these minerals. Taking these figures 

 for what they are wortk, we find 3*996 per cent, of soda and 

 4'516 per cent, of potash. This gives a molecular ratio for soda 

 slightly higher than for potash. 



In the slices examined for the purposes of this paper it was 

 estimated that the alkali-felspar and plagioclase exist in approxi- 

 mately equal proportions ; probably the former is slightly in excess 

 of the latter. This would bring the rock within the adamellite 

 category of Hatch. 1 Probably, however, the proportion of lime 

 in the plagioclase is very low, so that the rock may be provisionally 

 described as an alkali-granite. 



The occurrence of abundant perthite and several types of micro- 

 graphic intergrowth, together with a well-marked porphyritic 

 structure, in this rock opens up interesting questions as to the 

 origin of these structures and the physico-chemical laws which 

 have governed their formation. The porphyritic felspars are some- 

 what indefinite in outline, without sharp boundaries, and often 

 showing inclusions of the other minerals of the rock. It seems,, 

 therefore, unlikely that they can be of prior consolidation. All the 

 available evidence suggests that these phenocrysts were formed in 

 their present position concurrently with the final solidification of 

 the rock. The tendency of modern ideas as to the physics of 

 crystallization of rock-magmas is towards the belief that pheno- 

 crysts of this sort represent the excess of one component above the 

 eutectic ratio for the several components. In this case the felspar 

 molecule was in excess and crystallized first, while the finer ground- 

 mass, including the various forms of micropegmatite, represents the 

 eutectic of the remaining components. Here, however, a further 

 complication ensues from the fact that the bulk of the felspar is 

 perthite, which is itself also a eutectic of a special kind, with partial 

 miscibility of its components, orthoclase and albite. A micropeg- 

 matitic intergrowth of quartz and perthite must, therefore, be 

 regarded as a ternary eutectic, in which the mutual relationships of 

 the components are somewhat different, since quartz is immiscible 

 with either of the felspars, while orthoclase and albite can form 

 mixed crystals within certain limits. 



1 ' Classification of the Plutonic Eocks,' in ' Science Progress ' 1908, p. 246. 



