Vol. 68.] THE GEOLOGY OF MYNYDD-Y-GADEE. 361 



a succession of bare weathered faces, on which the included 

 fragments stand out with great prominence : these are of all sizes, 

 up to a length of 18 inches. The large blocks are not associated 

 together so as to form a coarse agglomerate, but occur sporadically. 

 The material of the blocks is, in the main, rhyolite, but pieces of 

 slate and grit also occur. (See PI. XXXVII, fig. 2.) 



IV. COMPAEISON WITH OTHER ArEAS. (P. L. & S. H. R.) 



Although it has long been known that in North Wales a great 

 volcanic series lies either in the Arenig or in the Llandeilo Series, 

 or in both, there are few places where its precise relations to 

 the graptolite zones have been determined. The most important 

 district for comparison with that which we have been describing is 

 the district of Arenig itself, which is situated only 15 or 16 miles 

 to the north-east, and has been described in detail by Mr. W. G» 

 JFearnsides.^ 



The main Volcanic Series of Arenig lies above the Didymograptus- 

 bifidus Shales, and seems to belong to the Llandeilian. Apparently 

 it corresponds approximately with our Ashy Series, and, like it, 

 consists chiefly of ashes and agglomerates. These are generally 

 andesitic in character, and it is only towards the top that rhyolitic 

 beds predominate. Associated with the ashes and agglomerates 

 are numerous sills and sheets of intrusive rock, but these are more 

 varied in character than at Dolgelly. Among them, for example, 

 is a group of hypersthene-andesites, but no rhombic pyroxenes 

 have been found in any of the rocks of the district here described. 

 There is, however, another group of andesitic dolerites which 

 approaches more nearly in character to the dolerites of Dolgelly. 



No lava-flows of older date than the Didymograptus-hifidus Zone, 

 have been described at Arenig. But ashy fragments occur in the 

 D.-bifidus Zone itself, and the Calymene Ashes, which lie between 

 the D.-hirundo and the B.-extensus Zones, may be contemporaneous 

 with our Rhyolitic Series. In any case, they show that volcanic 

 eruptions took place in North Wales during the Arenig epoch, and 

 Dolgelly may well have been closer to the seat of activity. 



Near Portmadoc, another area examined by Mr. Fearnsides,^ the 

 evidence is not sufficient to determine the exact horizon of the 

 volcanic rocks. 



A group of volcanic rocks which lithologically presents some 

 resemblance to the Rhyolitic Series has been described by Miss Elles'^ 

 in the neighbourhood of Conway. But the base is not seen, and the 

 palsBontological evidence indicates that the part examined belongs 

 to a considerably higher horizon than any of the rocks in the area 

 here described. 



It is to South Wales that we must turn, in order to find a volcanic 

 group which both in age and in character approaches the Rhyolitic 

 Series of Dolgelly. On Skomer Island and the adjacent mainland 



1 Q. J. G. S. vol. Ixi (1905) p. 608. ^ Ihid. vol. Ixv (1909) p. 169. 



2 Ibid. vol. Ixvi (1910) p. 142. 



2d2 



