Reports and Proceedings — Geological Society of London. 327 



(^) 



Glyn Valley 

 Series;. 





Glyn Grit and Limestone 



Ddolhir Beds, with PIn/Uopora Hisingeri, Rami- 

 pora Hochstetteri, Triniccleus seticornis, species 

 of Cybele, Cheirurus, Hemopleicrides, and 

 numerous Cystids, Corals, Brachiopoda, etc. ... 

 GYii])tolite Slates, Vfith. Dicelloc/raptus ekffans 



Bala. 



Gap. 



,(e) Pen-y-Graig Ash 



I {d) Bryu Beds, with Tetradella complicata, TUacops 

 apictdatas, Triniccleus concentriciis, Triplesia 

 Pandy I spiriferoides, Rnfinesquiiia ungula, etc. 



Series. ] (c) Craig-y- Pandy Ash ... ... 



(5) Teirw Beds, with Lingula tenuigranulata, 

 Bellerophon nodosus, Asaphus Foiuisi, etc. 

 V(«) Cwm-Clwyd Ash 



In the Pandy Series the beds a, e, and e consist essentially or 

 wholly of ashes and tuffs (no lava -flows having been detected). 

 Movements have often taken place along the two surfaces of the 

 Craig-y-Pandy Ash, and the rock is commonly foliated, and locally 

 converted into a white or blue china stone, in which good columnar 

 jointing is sometimes shown. The authors have failed to detect any 

 traces of thermometamorphic action of these bands on the adjoining 

 slates, such as has been recently asserted. It is otherwise with 

 an intrusive sill, which has been commonly mistaken for the upper- 

 most ash. 



The Teirw Beds are compared with the Poman Pell Group of the 

 Lake District. The Bryn Beds agree most nearly with the Sleddale 

 Beds of the same district. 



The Graptolite Slates are separated from the underlying Pandy 

 Series by the Ddolhir Fault, which has cut out a considerable part of 

 the succession. 



The Ddolhir Beds are extremely rich in fossils, which prove their 

 equivalence with the Ashgillian formation of other districts. They 

 appear to pass up into the Glyn Grit and Limestone, which is 

 essentially a sandy and calcareous facies of the series. The Glyn 

 (}rit corresponds in position with the Cor wen Grit ; but, recognizing 

 that it belongs to the Bala Series, the authors are no longer prepared, 

 without further evidence, to assume the equivalence of the two grits. 



The Pron-Prys Slates show the lithological characters of the Grey 

 Slates of Corwen, but belong rather to the shelly than to the 

 graptolitic facies of the Llandovery Series. 



The Ty-Uraw Slates resemble the pale Tarannon Slates of other 

 districts. They appear to pass up into the Denbighshire Slates. 



The authors find no indication of the overlap or overstep of the 

 "Wenlock, Tarannon, and Llandovery Beds mapped by the officers of 

 the Geological Survey, or described by previous observers, although 

 there is probably an unconformity at the base of the Pron-Prys Slates. 



The beds of the district dip northwards at an almost uniformly low 

 angle, but the structure is considerably complicated by a series of 

 faults, most of which have hitherto escaped notice, some being very 

 elusive. They include — (1) east and west faults, (2) north-north- 

 west and south-south-east, or north and south faults, and (3) north- 

 north-west and east-south-east faults. 



