Beporfs and Proceedings — Geological Society of London. 331 



followed l>y Pleistocene and recent deposits; 80 that they probably 

 ranoe from older to newer Pliocene in date. 



Part ii contains the observations of Mr. Rutley on the petrology 

 of the rhyolites. The rocks present occasional occurrences of 

 perlicity, and the lithoidal types sometimes owe their characters to 

 subsequent devitrification, sometimes to the eifect of cooling on or 

 immediately after eruption. Reheating has at times reduced the 

 felspars to the condition of fels|)ar-glass. Although plagioclase- 

 felspar is common, the analyses indicate that the series must be 

 retained with the rhyolites, it being quite possible that some of 

 these minerals may have been derived from the andesites. In the 

 rocks of Omahu, branching spherulites of microfelsitic material 

 are described in detail, the microfelsitic substance being an 

 imperfectly devitrified glass of fil)rous aspect, the fibres having an 

 imperfectly margaritic or longulitic cliaracter. 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 : " wliile 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., R.N. 

 (Communicated by Sir A. Geikie, U.Sc, LL.D., F.R.S., F.G.S.) 



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 quai'tzites). 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 sills 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 nietamorphism 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 loiotite, 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, 



