part 4] THE SCHISTS OF THE SCHICHALLION DISTRICT. 427 



III. The Beds between - the Quartzite and the 

 Stritan Flags. 



North and east of the bounding strip of Boulder Bed, it is 

 easy to distinguish a series of rock-types, which I believe to form 

 a stratigraphical sequence in continuation of that which has been 

 already made out. These are the four lowest members of the 

 following table, which is intended to represent the succession in an 

 area extending from the Schichallion district to the south-eastern 

 corner of Sheet 55 of the Survey map. That part of the series 

 which is present near Schichallion is included in the hirger bracket. 



Ben Ledi Grits. 



Green Beds. 



Garnetiferous mica-schist (Pitlochry Schist). 



Loch Tay Limestone. 



Garnetiferous mica-schist (Ben Lui Schist). 

 | Ben Lawers Schist. 



Ben Eagach Schist. 



Cam Mairg Quartzite. ~) 



Killiekrankie Schist. I Quartzite 



Schichallion Quartzite, with f Group, 

 intercalated Boulder Bed. J 



Main Boulder Bed. 



White Limestone. 



Banded Series. 



Grey Limestone. 

 ^ Grey Schist. 



The beds not included in the bracket, and the succeeding mem- 

 bers as far as the Quartzite Group, are in inverted order, when 

 compared with the previous table (p. 424). For reasons which 

 will afterwards be given, I think it likely, though not convincingly 

 proved, that the foregoing succession, when read upwards, is in 

 true chronological order. 



Grey Schist. — This is a pelitic or micaceous type, containing 

 both white and brown mica. It is often so coarse as to be definable 

 as a muscovite-biotite gneiss. The biotite is sometimes very 

 largely altered to chlorite, while the non-flaky constituents include 

 quartz, plagioclase (oligoclase to andesine), kyanite (sometimes 

 well developed), and inconspicuous garnet. Two slides have been 

 made of this rock in its coarser phase ; both contain finely dis- 

 seminated carbonaceous material, and one shows distinct fiakes of 

 graphite. The presence of this mineral would not be suspected in 

 the hand-specimen ; but, where the rock is finer, its graphitic 

 nature is sometimes more pronoimced. The Grey Schist must 

 have been, though to a less degree than the Ben Eagach Schist, a 

 carbonaceous sediment. Both these rocks were classed as ' Black 

 Schist ' in the Geological Survey map and memoir, but that name 

 is more appropriate in the case of the Ben Eagach Schist. The 

 two can easily be distinguished on Held evidence, by the much 

 more sparing occurrence of highly graphitic layers in the Grey 

 Schist, at least so far as the district here described is concerned. 



