﻿214 



PROF. S. J. SHAND OK 



[vol. lxxii, 



The rock chosen for analysis shows a combination of the cha- 

 racters of types 2 & 3 as above described — that is, it contains both 

 felspar-laths and hornblende-prisms, with magnetite. The average 

 diameter of grain is 0*01 millimetre, and the usual quartz- and 

 felspar-inclusions are present in comparatively small amount in the 

 hand-specimen, although well-rounded inclusions are very numerous 

 under the microscope. An apparent discrepancy exists between 

 the abundance of hornblende in the ground-mass of the rock and the 

 rather low figure for magnesia in the analysis ; but it is possible 

 that the place of magnesia is largely taken by ferrous oxide, and 

 that the amphibole is rich in the grungrite molecule. 



The norm of this rock, as calculated from the first analysis, is as 

 follows : — Quartz 9"72, orthoclase 37 - 81, albite 28"82. anorthite 

 11*12, diopside 5 - 02, hypersthene 1*49, magnetite 3'71. The rock, 

 therefore, falls into persalane, subrang pulaskose (I, 5, 2, 3). 



Conclusions to be drawn from the analyses. — It was 

 recognized from the beginning that a chemical analysis would 

 probably prove inconclusive as regards the origin of the veins, 

 because (a) it is not possible to separate the ground-mass of the 

 rock from the minute granitic inclusions which occur all through 

 it, and (b) whatever theory of the origin of the veins one may 

 favour, it is certain that some part of the granite — especially 

 the biotite. but also some of the felspar — has already been in- 

 corporated in the ground -mass. The analysis, therefore, gives the 

 composition of the ground-mass plus that of innumerable quartz- 

 and felspar- in elusions. 



No analysis has been made of the granite-gneiss in which the 

 veins occur : this is a composite banded gneiss in which different 

 bands show a very wide range of composition, and no single 

 analysis could give a fair idea of the average composition. From 

 the mineralogical examination of the various facies of the gneiss, 

 however, one may describe the rock-complex as a whole as having 

 the composition of a granodiorite. The real utility of the above 

 analyses, therefore, lies in showing that the composition of 

 the pseudotachylyte is such as might result from the 

 commingling of fragments derived from the different 

 leucocratic and melanocratic bands of the gneiss. 



My original assumption, that the dykes consist of basic igneous 

 material (approximately of the composition of basalt) which has 

 acidified itself by solution of granite, is now seen to be scarcely 

 tenable. The figures for alkalies and magnesia, in particular, are 

 such as no mixture of granite and basalt could give. Again, in 

 order to account for the proportion of silica alone, it would be 

 necessary to assume that our hypothetical basaltic magma took up 

 in solution and suspension from one and a half to three times its 

 own weight of granite, since, according to Dr. Daly's figures, the 

 average silica of basalts is 49 per cent, and that of granodiorites 

 and granites from 65 to 69 per cent. The actual proportion of 

 suspended granite-fragments (that is, inclusions) in the analysed 



