362 ME. p. LAKE AND PEOF. S. H. EETNOLDS ON [Sept. I912, 



'an interesting series of volcanic rocks has been described by 

 Mr. Herbert H. Thomas.^ The evidence of its age is not con- 

 clusive, but from a comparison with neighbouring areas Mr. Thomas 

 is inclined to place it in the D.-extensus Zone. The group consists 

 in part of rhyolitic lavas, and these are very similar in character 

 and constitution to those of Dolgelly. In both cases the flows are 

 sometimes banded and sometimes nodular. The general resem- 

 blance in character and probable age led us to have analyses made 

 of two of our specimens (see p. 360), and these analyses show that 

 there is an equally close resemblance in chemical composition. 

 Both at Skomer and at Dolgelly the rhyolites contain a high per- 

 centage of soda. 



But here the resemblance ends. The rhyolites do not form by 

 any means the largest part of the Skomer Series, and the Dolgelly 

 rocks are petrologically far less varied than those of Skomer. If, 

 however, the two series should really prove to be of the same age, 

 the similarity of the rhyolitic lavas will be of considerable interest. 



EXPLANATION OF PLATES XXXV-XXXIX. 



Plate XXXV. 



Geological map of the neighbourhood of Mynydd-y-G-acler, on the scale 

 of 3 inches to the mile, or 1 : 21,120. 



Plate XXXVI. 



Fig. 1. Fine-grained dolerite penetrating the coarser variety. The hammer 

 rests on one vein about 2^ feet wide ; and a second vein, 

 separated from the first by a narrow band of coarser dolerite. is 

 seen on the right. (See p. 353.) 

 2. Blocks of rhyolite caught up by, and enclosed in, the dolerite. 

 (See p. 353.) 



Plate XXXVII. 



Fig. 1. Nodular rhyolite, Mynydd-y-Gader. In the upper part of the 

 figure the sinuous flinty ridges characteristic of some parts of 

 the nodular rhyolite are seen, (See p. 359.) 

 2. Tuff's, Mynydd-y-Gader. (See pp. 360-61.) 



Plate XXXVIIL 

 Figs. 1 & 2. Banded rhyolite, Mynydd-y-Gader. (See p. 360.) 



Plate XXXIX. 



Figs. 1^. Nodular rhyolite, Mynydd-y-Gader. (See p. 359.) 



In fig. 2 one of the nodules shows a concentric structure and 

 central hollow. 



Q. J. G. S. vol. Ixvii (1911) p. 175. 



