192 DK. WILLS AND ME. SMITH ON THE LOWEE [vol. Ixxvili, 



(C) Silurian : (a) The Valeiitian Formation. 



Following with apparent conformity on the grits that constitute 

 the summit of the Ordovician, is a series of cleaved, usually 

 unfossiliferous, pale grey-green mudstones and shales ^ comprising 

 the Llandovery and Tarannon ' pale slates ' of earlier writers. 

 These can be conveniently grouped as the Yalentian Formation, 

 and may be subdivided as follows : — 



Upper Valentian or Tarannon (of Tarannon). 

 Lower Valentian or Birkhillian. 



Dr. Grroom & Mr. Lake studied these rocks first at Corwen,- and 

 later at Glj^n-Ceiriog,^ where they gave the local names of 'Ty- 

 draw Slates ' to the Tarannon and ' Fron-Frj's Slates ' to the 

 Lower Yalentian (their ' Llandovery ') . "We do not propose to adopt 

 these names, because the sequence at Griyn-Ceiriog is very attenu- 

 ated, and often shows signs of having been severely dislocated. It 

 is probable that part of the Yalentian is missing here, as a result 

 of strike -faulting. 



The fullest development of the Yalentian in the region under 

 description is brought to the surface by the anticlines of Cyrn-y- 

 Brain and Myn^^dd-Cricor. This is summarized in Table III, 

 opposite. 



In the Cja-n-y-Brain area, the rocks are well exposed, but highly 

 cleaved and much disturbed, and near Plas-uchaf, the eastern part 

 of the World's-End Fault cuts out much of the lower series, and 

 in its western part much of the Tarannon. 



The palaeontological evidence is very scattered. The chief data 

 may now be given. 



Lower Yalentian. 



The gritty grey-green mudstones with brachiopods are best seen 

 in an old opening 130 yards north of Tai-newyddion, on the Ruthin 

 road. Here were found : — 



Orthis (Hebertella) sp. 

 Plectamhonites cf. diiplicata J. de C. 



Sowerby. 

 Plectamhonites cf. transversalis 



Wahl. 

 Plectamhonites sp. nov. 

 Plectam,honites sp. 

 TrHobite. 



Atrypa reticularis Linnaeus. 



cf. Meristina furcata J. de C. 



Sowerby. 

 Orthis (Bilohites) hiloha Linnaeus. 

 07-this calligramma Dalman. 

 Orthis fDalmanellaJ elegantula (?) 



Dalman. 

 Orthis (Hebertella) protensa J. de 



C. Sowerby. 



This assemblage appears to represent a mixture of Lower and 

 Upper Llandovery faunas ; and, as elsewhere in the Llangollen 

 district, is not very helpful for purposes of correlation. The rocks 



^ Where the rocks are referred to as shales or mudstones, the cleaved 

 equivalent is implied. 



2 Q. J. G. S. vol. xUx (1893) p. 426. ^ Ibid. vol. Ixiv (1908) pp. 553-54. 



