502 ME. W. B. R. KING ON THE [vol. lxxix, 



The Glyn Grrit fossils do not appear to be the same as those 

 from the Meristina-crassa Sandstone, and it would seem that the 

 grit was being formed at a time when the Marchnant Valley was 

 actually above water, or at any rate undergoing submarine erosion. 

 The M.-crassa Sandstone would be the deposit formed on re- 

 submersion or renewal of sedimentation, and therefore correspond 

 in time with the upper part of the Corwen-Glyn Grit or probably 

 somewhat later. 



(c) Bala. — In many respects the correlation with the Bala 

 district is the easiest to make, for the faunal groups suggested by 

 Miss G. L. Elles are found to be applicable. Among the points of 

 detail, the fact that the ash-beds in the South- Western Berwyn 

 Caradocian are much fewer and thinner is worthy of note. 



The lithological type of the main mass, as well as the fauna, ot 

 the O.-alternata Sandstones is extremely similar in the two areas. 

 In the O.-actonicd Beds, however, the similarity is less marked. 

 In the lower portion, in the Marchnant Valley, there are only 

 slight signs of contemporaneous volcanic activity ; but the palseon- 

 tological evidence leaves no room for doubt as to the correlation 

 with part of the calcareous-ash group at Bala. At that locality, 

 however, there is no black graptolite- shale group corresponding to 

 the Pen-y-garnedd Beds ; presumably, if the theory of the lagoon 

 origin of the black shales be correct, the Bala area lay outside the 

 barrier which separated the open sea from the lagoon waters. 



In Ashgillian times conditions were again equalized ; but no 

 limestone comparable with the Rhiwlas Limestone was deposited 

 in the Marchnant Valley. 



Some doubt may be expressed as to the correlation higher in the 

 sequence, for no fossils comparable with those from the mudstones 

 of the Marchnant Valley have been found in the Moel-fryn Sand- 

 stones of Bala. The M.-crassa Sandstone appears to be about the 

 horizon of the somewhat more open-water beds represented by the 

 Hirnant Group. 



VI. General Summary of the Sequence of Events 

 in the South-Western Berwyns. 



During early Caradocian times a shallow sea covered the district, 

 which was gradually sinking to keep pace with the accumulation 

 of the sands and silts that were being brought into the region. At 

 one period volcanic ash from the north-west reached this part of 

 the Berwyns in sufficient quantities to form a definite ash-bed, 

 while at other times much fine volcanic material was mixed with 

 the normal terrigenous sediments. . 



The fauna at this period consisted largely of brachiopods, together 

 with a few trilobites. In later Caradocian times the sedimentation 

 was more calcareous, and again fragments of volcanic ash (mainly 

 in the form of bits of felspar) reached the area ; life was extremely 

 abundant in the shallow seas, the brachiopods and trilobites being 



