20 



MR. E. H. CUNNINGHAM-CRAIG ON 



[Feb. 1904, 



is evident, however, that in the area north-west of Inveruglas the 

 dynamic metamorphism does not diminish, and it may have increased. 

 The constructive metamorphism increases rapidly. 



Three specimens from the eastern side of the loch at Inversnaid 

 supply the next link in the chain. 



The first of these (8999, fig. 2, below) is a very siliceous rock, con- 

 sisting chiefly of folia of granulitic quartz with biotite, chloritized 

 biotite, albite, and magnetite. The quartz is all recrystallized into 

 a coarse mosaic, and the albites are larger and more distinct than in 

 the last specimen ; there is also a tendency for the albite-grains to 



Fig. 2.— Slide No. 8999. 



[Seen under a 1-inch objective.'] 



[A highly-crystalline gneiss from Inversnaid, showing the first appearance of 

 small authigenic albites and the leaching-out of quartz into lenticles. 

 Muscovite and biotite are also present.] 



be concentrated in folia. In a more micaceous specimen (9000), 

 chlorite and muscovite are very abundant, being associated both 

 with folia of quartz and with the authigenic albites ; but the 

 albites are hardly, if at all, more abundant than in the more 

 siliceous specimen.. The third specimen (3680) is composed chiefly 

 of flakes of chlorite and muscovite lying at all angles to the bedding, 

 with interstitial quartz and a few large albites. Thus, in a rock 

 which most closely resembles a phyllite or slate in composition, the 

 development of albite is little greater than in a highly-quartzose 

 gneiss evidently formed from a grit. The albites in these specimens 

 show little or no trace of idiomorphic outlines : they appear as rounded 

 or elongated grains. Polysynthetic twinning is never observed ; 



