128 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



and western portions of the area they have forced themselves 

 among the Lower-SUurian sedimentary strata that underlie the 

 Bala volcanic group — a position analogous to that taken by the 

 corresponding sills of the Arenig series. But they likewise invade 

 the volcanic group itself. Along the eastern borders of the district 

 they abound, especially in the higher parts of the volcanic group, 

 where they have been injected between the flows and have subse- 

 quently participated in the abundant plication of the rocks between 

 the mountains and the line of the Kiver Conway. 



The curvatures into which the rocks of the region have been 

 thrown, and the consequent breadth of country over which the 

 volcanic sheets can now be examined, furnish a much better field 

 than in Merionethshire for the attempt to trace the probable centre 

 or centres from which the basic magma was protruded. A study of 

 the Survey maps soon leads to the conviction that the intrusions 

 were not connected, except perhaps to a trifling extent, with the 

 great line of western vents. It is remarkable that the older strata 

 which emerge from under the volcanic group on its western outcrop 

 are, on the whole, singularly free from sills, though some conspicuous 

 examples are shown opposite to Mynydd-mawr, while a few more 

 occur farther north along the same line. Their lenticular forms, 

 their short outcrops, and their appearance on difi'erent horizons at 

 widel}' separated points seem to indicate that the sills probably 

 proceeded from many distinct subterranean pipes. Their greater 

 abundance along the eastern part of the district may be taken to 

 indicate that the ducts lay for the most part considerably to the 

 eastward of the line of western vents. They may have risen in 

 minor funnels, like that of Capel Curig. 



It is remarkable that so abundant an extravasation of basic 

 material should have taken place without the formation of numerous 

 dykes. We have here a precise repetition of the phenomena that 

 distinguished the close of the preceding Arenig volcanic period in 

 Merionethshire. As the intrusion of an enormous amount of an 

 igneous magma into the Earth's crust may thus take place without 

 the formation of dykes, it is evident that the conditions for the 

 production of sills must be in some important respects difi'erent 

 from those required for dykes. 



No evidence has yet been obtained that any one of these sills 

 established a connexion Avith the surface. JN^ot a trace can be 

 found of the outpouring of any such basic lava-streams, nor have 

 fragments of such materials been met with in any of the tufis. On 



I 



