Ixxviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol. Ixxiii, 



elsewhere, for Dr. Cox has recognized as spilites the thick group of 

 lavas which forms the summit of the mountain, and with these 

 may probably be connected the intrusive sill of soda-granophyre 

 which makes so prominent a feature. 



The intercalation in the volcanic sequence of sills which were 

 intruded almost contemporaneously is a noteworthy characteristic 

 of Ordovician vulcanicity, whether of alkaline or of calcic facies. 

 It is illustrated by sills of ceratophyre at Abercastle and perhaps 

 of spilite on Mullion Island, but also by h}^)ersthene-andesites in 

 the Arenig hills and Shropshire and by rhyolites [in the Lleyn 

 district. Again, some plug-like intrusions breaking through the 

 Cambrian of Caernarvonshire not improbabh^ mark the sites of 

 Llandeilian volcanoes. With these exceptions the ver}^ numerous 

 intrusions in the Welsh area are later than all the volcanic 

 eruptions. Of a distinct plutonic phase of activity there is but 

 slight indication, and that only in the north-west, the diorite- 

 granite complex of Sarn being the chief representative. 



The minor intrusions are found in all the districts, but on the 

 whole in greater force as we pass towards the north or north-west, 

 and this is especially evident in the case of the acid rocks. More- 

 over, in this dii-ection — that of increasing stress — the concordant 

 habit gives place more and more to the transgressive. The intru- 

 sions, like the volcanic outbreaks, are sharply limited by the line 

 of the Llanberis ridge. The small Llanfaglen mass, if it belongs 

 to this age, is the onlv exception. The succession of different 

 types, in so far as it can be ascertained, is one of increasing 

 basicity. Some of the latest intrusions are proved in Shropshire to 

 invade the Peiitamerus Limestone of the Llandovery Series. Else- 

 where the numerous intrusions of dolerite are intercalated in the 

 main volcanic group and at lower horizons, but the}^ must certainl}^ 

 be referred to a late date. While affecting the sill habit, they do 

 not make sills with any great lateral extension, but tend to occur 

 along anticlinal and synclinal axes. We may infer that they were 

 injected at a time when folding of the strata was already beginning, 

 name!}' : that folding which culminated towards the close of the 

 Silurian in the main Caledonian crust-movements. 



Next to Wales the most im])ortant area of Ordovician igneous 

 activity is the Englisji Lake District. Both here and in the 

 neighbouring inlier of Edenside early eruptions are represented 

 by a few thin lavas and tuffs in the upper part of the Skiddaw 

 .Slates. The rocks, which are much altered, have been described 



