Ixxvi PEOCEEDIXGS OF THE GEOLOGICAL SOCIETY. [vol. Ixxiii, 



of Dolgellv are much albitized, and appear to be spilites. The 

 crystal-tuffs and agglomerates of Rhobell Fawr, immediately 

 underlying the base of the Ordovician, resemble those of the 

 Sanquhar district of Dumfries-shire, and will probably be found 

 likewise to have alkaline affinities. Later came the soda-rhyolites 

 of Dolgellv and Cader Idris, assigned by Mr. P. Lake & Prof. 

 S. H. Reynolds to a LoAver or Middle Arenig age, and on the 

 other side of the Harlech dome Prof. 0. T. Jones has noted spilite- 

 lavas in the Lower Arenig of the Lleyn. 



So far. the only important crust-movements had been of the 

 nature of a slow subsidence. The igneous action connected with 

 the subsidence shows a certain symmetrical disposition relative to 

 the shape of the geosynclinal region, and the general alkaline 

 nature of the igneous rocks is significant in the same sense. 

 Towards the close of the Arenig age, however, a change in all these 

 conditions begins to be evident. It is clearlv related to the coming 

 in of a certain element of lateral thrust, a premonition of that 

 which attained a much greater intensity at a later time. The 

 symmetry of arrangement gave place to a decided unilateral 

 disposition. By the accentuation of old features and the develop- 

 ment of some new ones, the original geosyncline came to be divided 

 into areas which can for many purposes be considered separately, 

 and in each of them the unilateral factor, related to a thrust 

 coming from the south or south-east, is apparent. The vigorous 

 outbreak of igneous activity which ensued was comprised mainly 

 in Llandeilo time, and its products are found to dift'er significantly 

 fi'om those of the earliei' eruptions, the spilitic series giving way 

 largely to the andesitic. 



I vnW consider especially the Welsh part of the region, since 

 there our information is relatively full. It was in some measure 

 isolated by the presence, on what may be tei'med its * lee side,'-of 

 the old massif of Anglesey. In front of this, as we now see it, 

 and having the same north-easterly and south-westerly direction, 

 are the two parallel ridges in which the younger pre-Cambrian rocks 

 emerge, one extending from Bangor to Caernarvon and the other 

 passing near Llanberis. The initiation of these ridges seems to 

 have been an early consequence of the slowly gathering thrust, 

 and they played the part of outworks to the Anglese}' barrier. A 

 broader uplift, with the same Caledonian trend, was developed 

 through Shropshire and Radnorshire, and, though too gentle an 

 undulation to act as a buttress, probably had its infiuence on the 

 distribution of eruptions in the area. 



