Ixxx peoceedexctS of the geological society, [vol, Ixxiii, 



the Lake District is represented in the Southern Uplands of Scot- 

 land, although local unconformity indicates some crust-movement 

 in that area at about the same time. Such vulcanicity as the 

 Llandeilo there shows seems to have been merely the continuation 

 of the Arenig outbreak. In Ireland early Ordovician eruptions 

 appear to have been confined to the marginal parts of the area. As 

 regards the interior, there are records of igneous rocks assigned to 

 the Llandeilo and Bala at numerous places from County Down to 

 the coast of Waterford ; but our knowledge, both on the strati- 

 grajDhical side and on the petrographical, is still so imperfect that 

 general conclusions would have but little value. 



Dm-ing the Bala and part of Silurian time there was renewed 

 subsidence, affecting more or less the whole of the area, but a 

 complete restoration of the early Ordo^^cian conditions was not 

 now possible. Crustal stress, not wholty relieved by the Llan- 

 deilian vulcanicity, was doubtless unevenly distributed, and was 

 undergoing change. We find accordingly that igneous activity 

 during this time was feeble and sporadic, and showed no common 

 characteristics of the petrographical kind, though a reversion, or 

 partial reversion, to sodic types is often noticeable. Setting aside 

 such areas as the Shelve district and that of the English Lakes, 

 where Llandeilian vulcanicity was prolonged into Bala time, there 

 were revivals during the latter age at a number of isolated centres. 

 In the Southern Uplands there was an outbreak in the neigh- 

 bom-hood of Peebles, the lavas being now of acid instead of basic 

 natm*e, but still of sodic types. In the Berwyns again the alka- 

 line nature of the Bala igneous rocks is unmistakable ; but in some 

 other cases, such as the neighbouring Breidden Hills and the 

 Kildare inlier in Ireland, the petrographical facies is not clearly 

 characterized. These Bala revivals, if we except that of Peebles- 

 shire, were all in the interior of the area, but the scattered out- 

 breaks in the Silurian occm'red in the marginal belt, those assigned 

 to the Upper Llandoveiy being in the neighbourhood of the Bristol 

 Channel. Of these the Mendip and Toi-twoi-th lavas, although 

 described as andesites, seem to be of somewhat alkaline composi- 

 tion, and the same may be true of the basic lavas with ' pillow- 

 structure ' at Milford Haven. The latest volcanic rocks of this 

 suite, seen at Clogher Head in the far west of Ireland, are mainly 

 of acid t^'pe. 



The most highly alkaline of our Lower Palseozoic igneous 

 rocks, those of the Assynt district, I have perforce omitted. The 



