Ixxxviii PEOCEEDINGS OF THE (:^EOLOGICAL SOCIETT. [vol. Ixxui, 



Red Sandstone, and the Carboniferous. Of these the second was 

 calcic and the thii-d alkaline, while the first, as a result of the 

 premonitory distui'bance in mid-Ordovician time, partook of both 

 characters in tm*n. 



Other points of difference maj be noted. Tlie Lower Old Ked 

 Sandstone cycle, which was most closely bound up with the oj^era- 

 tion of lateral thrust, illustrates a well-defined type of igneous 

 action. All manifestations were localized within certain areas, 

 having a definite relation to contemporaneous folding. Explosive 

 outbm'sts and the extravasation of lavas constituted the first phase 

 of activity ; then came the intrusion of plutonic rocks in the form 

 of bosses ; and finall}^ if we disregard exceptional overlapping, the 

 injection of large nirmbers of dykes and sheets. In the volcanic 

 phase lavas of different kinds were poured out from neighbouring 

 centres. The plutonic rocks broke through in the midst of the 

 several volcanic districts, and sometimes in places already marked 

 out in the volcanic phase. The minor inclusions clustered about 

 the plutonic centres, and tended to orient themselves A\ith reference 

 to those centres. In the earlier and later igneous cycles this 

 orderly disposition is not seen : there were no fixed foci of activity, 

 the theatre of operations shifting from one district to another. 

 The plutonic phase was scarcely represented in the Ordovician and 

 wanting in the Carboniferous Period. With this there went a 

 certain confusion between the exti'usive and the intrusive phases, 

 shoAvn by the fi'equent interpolation of quasi-contemporaneous sill- 

 intrusions in the volcanic succession. 



In most of the history that I have briefly outlined Southern 

 England bore no j^ai't. Lying outside the region principally 

 affected by the Caledonian disturbance, this area presents so far a 

 much less eventful record. After the early Ordovician activity 

 had passed away from the border belt, there foUoAved here a long 

 season of quiescence ; and, when vulcanicity again broke out, its 

 petrographical facies was unchanged. The Upper Devonian lavas 

 of Noi-tli Cornwall and South Devon are spilites, together with 

 some soda-rhyolites in the Dartmoor district, and the associated 

 intrusions are mostly of albitized dolerites, with more basic tyjjes, 

 minverites and j^endotites. Further, when another revival of 

 activity took place in the time of the Lower Culm, alkaline tj^pes, 

 including spilites, recurred once more. 



It is interesting to remark that, during the earlier half of the 



