18 ME. E. H. CUNNINGHAM-CRAIG ON [Feb. IC)04 t 



quartz. The felspar-pebbles are almost completely destroyed and 

 decomposed, and are associated with the micaceous films. The 

 biotite is in well-developed flakes, lying at all angles to the 

 schistosity, but it seldom pierces the quartz-granules of the matrix, 

 showing that the quartz has not been recrystallized to any great 

 extent. The biotite is the first clearly authigenic mineral to be 

 observed in these rocks, and has evidently been developed by 

 constructive metamorphism after the shearing-movements which 

 produced the schistosity had ceased. 



From this point onward, as we recede from the Highland 

 Border, allothigenic minerals decrease in number, while there is a 

 corresponding increase in authigenic constituents. I am inclined to 

 regard the constructive metamorphism which has affected the last- 

 described specimen as probably a normal thermometamorphism. In 

 the specimens that follow we find effects, increasing to the northward, 

 of a different type of constructive metamorphism, the nature of 

 which will be discussed later. 



The next specimen (8985) Avas collected at Hudha Dubh, 

 1| miles to the north-north-west. It has probably been a finer- 

 grained rock originally, and occurring in a locality where the beds 

 are lying at a low angle, schistosity has reached a much higher 

 stage. The rock consists of irregular grains of quartz and plagio- 

 clase, with folia of sericitic mica, chlorite, and green or chloritized 

 biotite, some calcite (probably from decomposed plagioclase), and a 

 few grians of sphene and magnetite. Pebbles have disappeared, but 

 the arrangement of lenticular aggregates of granulitic quartz suggests 

 that pebbles may once have been present : they may be called the 

 ' ghosts ' of clastic grains. The quartz and felspar do not appear to 

 have been recrystallized to any extent ; but the micas, chlorite, and 

 possibly sphene, are authigenic. The presence of calcite, the deve- 

 lopment of chlorite, and the chloritization of biotite might be 

 attributed to weathering. I am inclined to regard them, however,, 

 as the first stages in the special type of constructive metamorphism, 

 which, from this point to the head of the loch, becomes increasingly 

 conspicuous. 



A rock (8986) from the shore opposite Tarbet Isle, where the 

 strata and folding are practically horizontal, carries the meta- 

 morphism a stage further. This is a siliceous but conspicuously- 

 schistose rock, with a considerable development of white mica, 

 which gives it a flaser-structure. Only the 'ghosts ' or suggestions 

 of original pebbles are to be seen, but their shape and size point to 

 the rock having been originally coarser in grain than the last. 

 Fragments of both oligoclase and alkali-felspar are present, although 

 they may be in part recrystallized. The quartz is certainly 

 becoming authigenic by recrystallization into a larger mosaic, 

 the grains of which not infrequently include flakes of biotite. 

 Some calcite and magnetite are also present. 



At this stage, it may be as well to glance at the evidence from the 

 area to the eastward of the loch, aud approximately on the same line 



