301 mr. e. b. bailey ox the [June 1913,. 



The limestones are generally black or dark blue, and are often 

 of very considerable thickness. In crystalline texture they vary 

 from fine to coarse, some occurrences being made up for the 

 greater part of crystals of black calcite a quarter of an inch across.. 

 Many beds are strongly pebbled, with large grains of quartz and 

 felspar, identical with those that occur in the Grin an Quartzite. 

 In such cases pebbles of fine-grained limestone, slate, and epi- 

 diorite are not infrequently found. The slates of the group 

 and the quartzite intercalations also not infrequently assume a 

 conglomeratic facies. 



Two conglomerates, the Glen Aray and the Loch-na-Cille Con- 

 glomerates, are particularly conspicuous, and have been indicated 

 on the appended map (PI. XXXII). One, the Glen Aray Con- 

 glomerate, has already been described in some detail in its outcrop 

 between Glen Aray and Loch Awe (p. 287). West of Loch Awe, 

 in the neighbourhood of Kilchrenan, it retains the same characters 

 as in Glen Aray, except that (so far as I could see) it no longer 

 carries epidiorite-fragments. The most abundant boulders are 

 pebbly quartzite, but Mr. Kynaston has made the very interesting 

 discovery of granophyre- and felsite -pebbles in it [16, p. 31]. 



The Loch-na-Cille Conglomerate, much farther south, was- 

 detected by Dr. Peach [18, p. 71]. It is extremely full of frag- 

 ments of slaggy epidiorite, and also contains numerous pebbles of 

 felsite, or porphyry, and quartz-syenite. Dr. Flett [18, p. 75] has 

 investigated the petrology of the pebbles, and has found that the 

 syenites are of the same type as those that occur in the conglo- 

 merates of Schiehallion, Islay, and the Isles of the Sea (Garvellach 

 Isles), long ago described and compared by Macculloeh [1, pp. 159 r 

 249]. It is an interesting circumstance that the source of these 

 quartz-syenite boulders is quite unknown. 



Further careful search might quite likely reveal the presence 

 of similar felsite, and perhaps even quartz-syenite, pebbles in 

 several other conglomeratic beds of the Loch Awe district. At 

 present, there are only two other instances of the kind known : 

 Mr. Kynaston has recorded felsite-pebbles from a conglomeratic 

 limestone, shown on the map (PI. XXXII) as an isolated outcrop 

 east of Loch Awe and 3 miles south of Kilchrenan [16, p. 31] ; 

 while Mr. Maufe has also found felsite in a conglomerate near 

 Ormaig, about a mile south of the head of Loch Craignish and 

 half a mile in from the coast [17, p. 37]. 



In the district dealt with in the foregoing pages these foreign 

 pebbles are all found in conglomerates within the Tayvallich 

 Division of the Loch Awe Group. It would be rash, however, to 

 assume that this statement holds true throughout the Highlands 

 and Islands of Scotland. 



The Loch Avich Slates and Grits. 

 In this division there are considerable masses of well-bedded, 

 green, cleaved mudstones ; these alternate with fine-grained 

 quartzose beds, sometimes containing sandy calcareous nodules 



