part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 175 



time-horizons, and that the innumerable, and often cacophonous, 

 local horizon-names would be dropped. 



The Author, in reply, thanked the Fellows for their generous 

 reception of her paper. In answer to Prof. Jones, she stated that, 

 with regard to the boundary between the Llandeilian and the Cara- 

 docian, she based her position on the palaeontological aspect of the 

 question. She regarded the rocks of the ' shelly ' facies of the 

 Welsh Ordovician as belonging to different faunas — the Ogygia- 

 selivyni fauna, the Placo])aria fauna, the Ogygia-huclii fauna, 

 the Calymene-planimarginata fauna, the JPhillipsinella-parahola 

 fauna, and the PJiacops-muc7''onatios fauna ; and, so long as beds 

 contained the same Ogygia-huclii fauna as those of the Llandeilo 

 area, she considered them Llandeilian in age. The beds with the 

 Nemagraptus-gracilis fauna, and even slightly higher graptolitic 

 horizons, were associated with the Ogygia-huclii fauna in the 

 Builth area, and were, therefore, in her opinion, of Llandeilian 

 age. 



With regard to the calcareous development of the Grelli-grin 

 Calcareous Ashes and the Rhiwlas Beds being usually separate as 

 in South Wales, that was not the case invariably in the Bala 

 country, since both occurred superposed at Bryn-pig. 



She had looked for evidence of faulting at Cwm Hirnant, but 

 could find none ; the beds were steeply inclined, yet along the old 

 adits there seemed to he a perfectly continuous section from the 

 Hirnant Beds to the shales with 3Ionograptus crispus. 



There certainly was evidence in the area north-west of Bala Lake 

 of movements similar to those on the south-eastern side ; these 

 appear to be related to a major thrust running at or near the 

 junction of the Dicranograptus Shales with the volcanic rocks. 

 She regarded the entire country as being thrust above the volcanic 

 series as a whole. 



In answer to Mr. Wedd, she stated that, while the cleavage in 

 the southern part of the area was inclined steeply east-south- 

 eastwards, when the effects of the east-and-west movement made 

 themselves felt in the northern part of the district, a second 

 cleavage seemed to be developed, the exact direction of which she 

 had not determined. 



She was in complete agreement with Prof. Watts in regard to 

 the growth of all these local names, which, however, were certainly 

 useful for local reference, but suggested the adoption of the 

 palaeontological classification of wide application for all other 

 purposes. 



