4<Q IGNEOUS AND ASSOCIATED ROCKS OF LLANWBTYD. [vol. lxxix, 



Dr. L. D. Stamp, in replying, thanked Prof. Cox for his kind 

 xernarks, and agreed that the thickening of the beds on the horizon 

 «of the Mydrim Limestone, as one went northwards from South 

 Wales, was very interesting. Llanwrtyd occupied a position 

 approximately half-way between the South Wales exposures (where 

 volcanic rocks are absent on this horizon) and Cader Idris (where 

 ;the volcanic rocks are so remarkably well developed). In reply to 

 Miss Elles, he said that the correlation of the Llanwrtyd grapto- 

 Uitic horizons with the Mydrim Limestone was based on the 

 "tabulated statements in the Geological Survey Memoir on the 

 Haverfordwest district. Only one of the Llanwrtyd species oc- 

 curred at all commonly on any other horizon in South Wales, 

 while all the others are shown as attaining their maximum develop- 

 ment in the Mydrim Limestone. With regard to Dr. Plymen's 

 Temarks, it was difficult to make any useful suggestions without 

 going more fully into the facts. 



Mr. S. W. Wooldeidge, in answer to Mr. Part's query, 

 described the occurrence of the chloritic pseudomorphs in detail ; 

 ibut stated his inability to identify the ferromagnesian mineral 

 vwhich had given rise to the chlorite. 



