. George Barrow — On certain Highland Gneisses. 65 



a broad dyke, belonging to the same original magma, and its 

 appearance is seen to be very different. The potash felspar is no 

 longer visible to the unaided eye, but it forms with the quartz a 

 finely granitic matrix, in which the earlier crystals of consolidation, 

 the two micas and oligoclase, are porphyritically embedded. Still 

 further, if the rock is slightly decomposed, the latter mineral, 

 somewhat tabular in form with conspicuous development of the 

 clinopinacoid, is seen to have an outer shell, fairly well marked off 

 from the much more rounded inner portion. The oligoclase obviously 

 came up in this form, and completed its growth from the magma in 

 which it lay, which later portion has been shown by Prof. Sollas ^ 

 to be more acid in character than the core. The round-grained 

 crystals then, of our round-grained gneiss, want their outer rim of 

 later growth. How is this to be accounted for? If such a granite- 

 magma be intruded during or towards the close of a powerful earth- 

 movement, it may be forced, by the tremendous pressure, into 

 every possible plane of weakness in the surrounding rocks. Now, 

 obviously, it is not the crystals, but the liquid portion of the rock 

 that must enter first, and all the crevices must be opened to the 

 diameter of the oligoclase crystals before any of them can enter. 

 The acid portion then of the magma, containing the constituents of 

 potash felspar, must travel somewhat in advance of the crystals of 

 earlier formation. In addition, the continuance of the pressure will 

 still further force the liquid from the solid crystals, leaving at last 

 just sufficient of the magma to fill the interstices between them. 

 Thus, in my opinion, has the perfect foi-m of round-grained gneiss 

 been produced, which is by no means uncommon. The more acid 

 magma will travel furthest, and finally consolidate as pegmatite. 

 Put back the pegmatite into the gneiss, and you have the compo- 

 sition of a normal granite. The round-grained character of the 

 oligoclase is thus seen to be due to the draining off of the magma, 

 from which it normally finishes its growth. 



To turn now to the question of the absence of selvage edges to 

 the gneiss. Obviously a coarse-grained intrusion of an inch in 

 thickness would only be possible if the country rock were nearly at 

 the same temperature as the magma. There is further evidence 

 tending to this conclusion, but as some parts of it are matters of 

 dispute, it may be omitted for the present. 



From the foregoing remarks we may draw the conclusion that 

 neither a round-grained gneiss of the above type, nor its accom- 

 panying pegmatite, is in any true sense a metamorphic rock ; both 

 consolidated as we now see them. In addition, it would be more 

 accurate to describe the pegmatite as being extruded from the gneiss, 

 than intruded into it. The latter view implies a later date, while 

 both are obviously of the same age. Like any other rock, such 

 gneisses and pegmatites are liable to subsequent metamorphism. 



^ See Prof. Sollas' paper, " Contributions to a Knowledge of the Granites of 

 Leinster," Trans. Eoy. Irish Acad. vol. xxix. p. 427. 



DECADE III. VOL. IX. NO. II. 5 



