(Jn Dalradian Sediments in the Region of Loch Awe. 397 



the rocks of Oniahu, branching spherulites of microfelsitic material 

 are described in detail, the microfelsitic substance being an im- 

 perfectly devitrified glass of fibrous aspect, the fibres having an 

 imperfectly margaritic or longulitic character. Among the fibres 

 often lie filmy scales which may possibly be tridymite. The term 

 microfelsite is considered worthy of retention, as it denotes 

 a phase of devitrification not adequately expressed by any other 

 name ; ' while its abandonment would produce a gap which could 

 only be bridged by much circumlocution.' 



4. ' On the Progressive Metamorphism of some Dalradian 

 Sediments in the Region of Loch Awe.' By J. B. Hill, Esq., E.N. 



The region under discussion contains two principal series of 

 rocks, passing one into another without a break, and conveniently 

 referred to the Dalradian System : — (1) The Ardrishaig Series 

 (phyllites and fine-grained quartzites). (2) The Loch Awe Series 

 (black slates, limestones, grits, and quartzites). The latter series lies 

 in a gentle trough of the former. Even in their most altered state, 

 the clastic nature of the rocks of the Loch Awe Series is apparent. 

 Both series are pierced by innumerable intrusive siils of epidiorite, 

 hornblende-schist, and chlorite-schist, modified diorites and gabbros, 

 which effect contact-metamorphism in the bordering sediments. 

 Intrusive rocks of post-schistose date also occur, like the Glenfyn 

 granite, the granite of Ben Cruachan, and smaller masses of 

 granite, monzonite, hyperite, ultrabasic rocks, quartz-porphyries, 

 felspar-porphyries, porphyrites, and lamprophyres ; these are in 

 their turn cut by dolerite and basalt- dykes. All these rocks 

 exhibit progressive metamorphism when traced towards the north- 

 east and towards the Central Highlands, a character best seen 

 in the loop formed by the rocks near the head of Loch Awe. The 

 Ardrishaig phyllites, almost clay-slates, pass into mica-schists of 

 the normal Central Highland types. The Loch Awe limestones 

 become epidotic, and contain biotite, garnet, and actinolite, the 

 crystals of the last mineral being sometimes 1 inch long. The 

 black slates acquire garnets and actinolite. The grits and 

 quartzites become granulitic, and pass into gneissose rocks with 

 black and white mica, garnets, actinolite, tourmaline, epidote, and 

 red felspar ; they pass into the quartzites, gneissose flagstones, 

 and biotite-gneisses of the Central Highlands, without anywhere 

 reverting to the approximately unaltered condition which they 

 exhibit to the south-west. The sills become granulitized and 

 partly recrystallized, with biotite and garnets. 



These rocks, which occur on the strike of the Central Highland 

 schists, are linked with the latter by the persistence of the Loch 

 Awe rock-types, including the bculder-bed, which is regarded as 

 having a definite stratigraphical position. It is true that when 

 traced along the strike the metamorphism increases rapidly towards 

 such granite-masses as that of Ben Cruachan, but the same type of 

 metamorphism continues into the Central Highlands, and into 

 regions so remote from the granite that the author is driven to 

 believe that ' without the presence of the Cruachan granite the 



