IXXIV PROCEEDIXaS OF THE GEOLOGICAL SOCIETY. [vol. IxXlii, 



uniform in rate, and was at times interrupted and loeallv reversed. 

 In particular, the Ordovician conditions were ushered in bv differ- 

 ential vertical movements, which accentuated the geosynclinal 

 structure, defining more strictly the north-western border, pro- 

 bably the southern too, and developing areas of relative elevation 

 in the interior, which were broadly of the nature of horsts. It 

 was about this time that igneous action was initiated, breaking out 

 in the marginal pai-t of the basin, and affording relief to the 

 differential stresses which there attained their maximum. 



Following upon a prolonged gentle subsidence, not complicated 

 by any movements indicative of lateml thrust, these early eruptions 

 had everywhere a pronounced alkaline, that is, sodic, facies. The 

 earliest, which broke out close to the actual borders, all belong to 

 the ' spilitic series ' of Dewey & Flett. The predominant basic 

 lavas are typical spilites, and, if acid types occur, they are soda- 

 rhyolites with corresponding tuffs. The associated intrusive rocks 

 are of types constantly found in this connexion, such as albite- 

 dolerites and soda-granite-porphp-ies. Here comes the volcanic 

 series of which relics are found along the actual Highland Border, 

 especially in Forfarshire and Kincardineshire. The volcanic belt 

 is perhaps prolonged fai-ther south-westwards, for to this series 

 Mr. Gunn has referred the North Glen Sannox spilites and Su* 

 Archibald Geikie the volcanic rocks of T^'rone. In Cornwall the 

 spilites of the Lizard and Gorran Haven seem to preserve relics of a 

 similar train of volcanic centres on the southern border of our area. 

 The age of the eruptions in these several districts cannot alwa^'s be 

 fixed with precision. They are commonly referred to the Arenig, 

 but on the Highland Border at least they may, according to 

 Dr. Campbell, be Upper Cambrian. 



From this narrow border -belt igneous acti^'ity shifted to a 

 ^vider belt lying somewhat within the border. The eruptions here 

 were on a grander scale. Dating from the Arenig, they continued 

 in some districts into part of Llandeilo time. In addition to the 

 spilitic series, the rocks of this submarginal belt include other types, 

 likewise characterized by richness in sodic felspars. Here belong in 

 the north the volcanic districts of the Southern Uplands of Scot- 

 land, extending from Girvan and Glenluce to the Muirfoot Hills 

 in Peebleshire, and in the west the Lower Palaeozoic inliers of the 

 Lough Mask district, described by Prof. Eeynolds & Mr. Gardiner. 

 It may be remarked that the plutonic complex of Girvan consists 

 of calcic types of rocks, and in the Irish district also the associated 



