A^^'IVEus\llv address op the tiiesident. 147 



Isles. Owing, however, to the constant plication of the strata, and 

 the wide space which the overlying sedimentary deposits arc thus 

 made to cover, the volcanic group only comes occasionally into view 

 and thus occupies but a mere fraction of the superficial extent oj 

 the region over which its scattered outcrops appear. These ex- 

 posures, sometimes only a few square yards in extent, may alw^ays 

 be looked for where the anticlinal folds bring up a sufficiently low 

 portion of the Silurian scries ; they prove that a vast volcanic 

 floor underlies the visible Lower-Silurian grits and shales over the 

 length and breadth of the southern uplands of Scotland. 



In the early days of the application of the microscope to the 

 elucidation of questions in field-geology I found some of the Lower- 

 Silurian shales of Moffatdalc to contain fresh felspar crystals, which 

 I felt tolerably certain had been supplied by volcanic explosions ; 

 and I used to speculate on the possibility of their having been 

 wind-borne from the volcanoes of the Lake District. I had at that 

 time no suspicion that their source was rather to be sought under 

 my feet. The presence of volcanic rocks underneath the uplands 

 of the south of Scotland may help to explain the frequent felspathic 

 composition of many of the Silurian greywackes and shales of that 

 region, and particularly the abundance of andesitic and felsitic frag- 

 ments in them. 



Without anticipating the details which will properly appear in 

 the Memoirs of the Survey, I may briefly indicate the visible 

 boundaries of the volcanic group and refer to some of the localities 

 where it may best be seen. The most easterly points where it has 

 been recognized by Mr. Peach stand on the crests of some sharp 

 anticlinal folds near St. Mary's Loch and near Leadburn in Peebles- 

 shire. Farther westwards it appears at many places along the 

 northern border of the Silurian territory, as at Eomanno Bridge. 

 Kilbucho, Culter AVater, and Abington, the length and breadth of the 

 exposure depending partly on the breadth of the anticline and partly 

 on the depth to which it has been cut down by denudation. Near 

 Sanquhar it opens out for a breadth of more than a mile, and is seen 

 at intervals across the wild moorlands o£ Carrick, until from the 

 Stinchar valley it widens out seaw^ard and occupies most of the 

 coast-line of Ayrshire between Girvan and the mouth of Loch Ryan. 

 It probably rises again along a fold near Port Patrick, and it is seen 

 at various points along the southern borders of the Silurian uplands, 

 as near Castle Douglas, at Glenkilu, Bell Craig near Mofi'at, and 

 the head of Ettrickdale. 



