198 Geological Society : — 



Ordovician. 



Bala. Ashgillian. Glyn- Corwen Grit. 



Dolhir and Cyrn-y-Brain 



Beds. 

 Ty'n-y-twmpath Beds. 

 Caradocian. Blaen-y-cwm Beds. 



Bryn Beds and Ashes. 

 Teirw Beds and Ashes. 



A revision of the conclusions of previous workers in the district 

 south of the Dee and a brief description of the rocks on the 

 north are attempted, with the following main conclusions : — 



(1) The gap in the sequence proved by Dr. T. T. Groom & 



Mr. P. Lake between the Ashgillian and the Caradocian 

 in Glyn Ceiriog, is filled farther west by the Ty'n-y- 

 twmpath Beds with an Ashgillian trilobite -fauna, and by 

 the Blaen-y-cwm Beds with graptolites, probably of the 

 Diplograptus-pristis Zone. 



(2) The Dolhir Slates and the fine-grained part of the Grlyn 



Grit of Glyn Ceiriog is replaced by a more sandy type 

 on Cyrn-y-Brain and Mynydd-Cricor with a large 

 brachiopod fauna, Ileristina crassa being the characteristic 

 form. This, formerly regarded as a Llandovery species, 

 is associated with Ashgillian trilobites. 



(3) The conformity of Silurian and Ordovician is confirmed. 



(4) A fuller Yalentian sequence has been found north of the Dee 



than that known at Glyn Ceiriog and Corwen. Though, 

 as a whole, unfossiliferous, graptolites indicate that the 

 Lower Valentian includes the Cyphus and Convolutus 

 Zones. Between them and the Tarannon Series there 

 appears to be a non-sequence. 



(5) Probably all the zones of the Tarannon Series of Tarannon 



are represented. 



(6) TheWenlock Series is comparatively thin, and consists of: — 



Unfossiliferous slates and sandstones. 

 Pen-y-glog Grit, in the western part only. 

 Bastard Slates with Cijrtograptus linnarssoni. 

 Eoofing Slates with Monograptus riccartonensis. 

 Slates and mudstones with Cyrtograptus murchisoni. 



(7) The Ludlow Series is rather unfossiliferous, but the following 



zones appear to be represented. 



Dinas Bran Beds. Dayia navicula. 



Yivod Beds. Monograptus leinhvardinensis. 



C ? Monograptus tumescens. 

 Nant-y-Bach Beds. < Monograptus scanicus. 



(. Monograptus nilssoni. 

 Glyn-Dyfrdwy Beds. Monograptus nilssoni (part). 



The Lower Ludlow and the Wenlock Series cannot satisfactorily 

 be separated. 



The Lower Palaeozoic rocks are, in the main, folded on approxi- 

 mately east-and-west axes, to which the cleavage and some of the 

 major faults are closely parallel in direction. The folding and 

 part of the faulting are Devonian, and therefore related to the 

 Caledonian movements, which, in the present case, appear to have 

 set up torsional stresses affecting a greater area than that considered 



