Vol. 69.] LOCH AWE SYNCLINE (ARGYLLSHIRE). 305' 



and layers, and also with thoroughly pebbly quartzites or grits 

 like those of the Crinan Division. It is difficult to form an 

 opinion as to the relative abundance of slates and siliceous rocks 

 in the group as a whole, but I think that the former are in excess. 

 The grits are not infrequently conglomeratic. 



The Tayvallich Volcanic Zone. 



Dr. Peach's ' Tayvallich Volcanic Zone ' is situated within the 

 Tayvallich Slates and Limestones and the lower part of the Loch 

 Avich Slates and Grits. The lavas are distinguished by their 

 vesicular character, fine-grained, texture, and not infrequent 

 pillow- structure, together with their failure to induce contact- 

 alteration. Characteristic tuffs and agglomerates are more or less- 

 absent, but conglomeratic beds containing fragments of vesicular 

 epidiorite are common. Certain felspathic beds in the Tayvallich 

 Peninsula have been interpreted as crystal-tuffs. 



As already stated, Dr. Flett [18, p. 84] has made a careful 

 petrographical examination of the lavas. He has referred them to 

 the spilite group, and shown that their felspar is always albite ;. 

 their ferromagnesian constituent is chlorite, not hornblende. 



The Epidiorite Intr- 



usions. 



, The epidiorite sills are of doleritic texture, where not much- 

 sheared, and often show coarse ophitic structure in their central 

 portions. In not a few cases they obviously transgress the bedding 

 of the associated sedimentary schists, and also very frequently 

 produce remarkable contact-alteration. 



Under the microscope they are seen to consist essentially of 

 hornblende and plagioclase, which Dr. Flett has found to be albite. 

 The presence of chlorite, with much carbonate, in the lavas, and of 

 hornblende, with comparatively little carbonate, in the intrusions,, 

 has led Dr. Flett to the view that the lavas were already much 

 decomposed before they were folded, while the intrusions were 

 fresh [18, p. 88]. 



Occasional leucocratic varieties are met with among the intrusive 

 epidiorites. Their petrology has been described by Mr. Kynaston 

 [16, p. 44] and Dr. Flett [17, p. 53, & 18, p. 91]. In one case 

 micropegmatite was found to occur locally ; in other instances the 

 intrusions proved to have bostonitic affinities. It must be realized 

 that these leucocratic rocks are extremely scarce. 



EXPLANATION OF PLATES XXXI & XXXII. 



Plate XXXI. 



Pillow-lava, An Aird, western coast of Tayvallich Peninsula. Eeproduced by 

 permission of tbe Controller of H.M. Stationery Office, from Dr. B. N. 

 Peach's account of the Tayvallich volcanic rocks [18, pi. iii, facing p. 59]. 

 (See p. 282.) 



