﻿Vol. 66.] AROTJtfD THE ROSS OF MULL GRANITE. 391 



present, but are very faint and not easily detected. Under a high- 

 power objective very minute inclusions are seen to be numerous. 



Much cordierite has been changed by weathering into golden- 

 brown pinite. Grains seen in the early stages of this change are 

 pale yellow, and show slight pleochroism. 



The following remarks are quoted from a letter sent to me by 

 Dr. J. S. Flett, F.G.S., when he had seen the slices of these 

 rocks : — 



' These are the most completely altered argillaceous rocks that I have ever 

 seen from the Scottish Highlands. The association of minerals is the same as 

 Dr. Teall * first described from Ben Cruachan, and Mr. Hinxman 2 found later 

 at Netherly in Elgin, in Moine Schists at the edge of " Newer Granites." The 

 included blocks in the Cornish granites contain the same group of minerals. 

 The only British rock that I know which contains such large crystals of silli- 

 manite is a specimen that Mr. Allan Dick has shown me from Croghan Domain, 

 north of Phillipstown, Sheet 109, Ireland. This seems to be Slide 2734, 

 described by Prof. Watts in his & McHenry's ' Guide to the Collections of 

 Eocks & Fossils,' Geol. Surv. Ireland, 1898, pp. 38 & 39.' 



IV. Kyanite and Tourmaline-Rocks ascribed to the 

 Regional Metamorphism. 



Mention has already been made of the abundant veins and 

 patches of pegmatite which are conspicuously older than those 

 connected with the granite, and participate in the folding of the 

 pelitic Moine rocks (p. 379). 



Around these quartz-pegmatite patches the pelitic gneiss is often 

 particularly coarse and quartzose, the quartzo-felspathic streaks 

 passing imperceptibly into the veins of ' pegmatite.' This streaky 

 quartzose character appears often in places in a sporadic way among 

 schists elsewhere containing but little quartz. There is, in fact, 

 every gradation between ' pegmatite ' veins and the ordinary quartz- 

 felspathic streaks proper to some of the coarser pelitic gneisses. 



Thus, although in these Moine gneisses the identity of the separate 

 beds of sandstone, mudstone, etc. has been maintained, there has 

 doubtless been some transfer of material from one part to another, 

 especially in the case of silica, and it seems likely that these 

 pegmatites are due to segregation of the more mobile constituents 

 present in the original sediments rather than to intrusion of material 

 from an underlying source. 



Prisms of tourmaline may be found in many of these pegmatite 

 patches throughout the area — pegmatites in no way connected with 

 the granite ; and in the two cases now to be described, the mineral 

 occurs along with kyanite as a constituent of the pelitic gneiss. 



In the first of these the kyanite occurs in the pelitic beds, within 

 a belt of the Moine rocks about 80 yards wide and traceable along 



1 Summary of Progress of Geol. Surv. for 1898, p. 85, etc. Also ' Geology 

 of the Country near Oban & Dalmally ' (Expl. of Sheet 45) Mem. Geol. Surv. 

 1908, p. 143. 



2 J. S. Flett, ' Geology of Lower Strathspey ' (Expl. of Sheet 85) Mem. Geol. 

 Surv. 1902, p. 52. 



