Yol. 62.] TARANNON SEEIES OF TARANNON. 645 



Hills certain ' pale-coloured earthy slates ' which * seem to pass 

 without a break into the overlying Denbigh Flagstone.' * From 

 the fact that they were aluminous and readily disintegrated these 

 'slates' were often referred to in the field by Sedgwick as the 

 ' paste-rock.' 2 At that time he and Salter regarded them as ; beds 

 of passage ' between the Cambrian and Silurian. 



In the year 1854, however, Sedgwick having arrived at the 

 conclusion that, in the Berwyn Hills and elsewhere, there was an 

 unconformity between his Cambrian and Silurian systems marked 

 by the absence of the May Hill Sandstone, relegated these ' paste- 

 rocks ' to a lower horizon, and placed them ' very nearly at the 

 crown of the whole Cambrian series,' 3 that is to say at the top of 

 the Bala formation. 



In the 1st edition of the Geological Survey maps of Wales (1850) 

 no attempt was made to show these 'pale slates' as a distinct 

 formation, and they were included in the ground coloured as Bala. 

 At a later date, however, Jukes & Aveline, 1 observing them to 

 pass naturally upwards into the Wenlock and Denbigh Beds, came 

 to the conclusion that they belonged rather to the Wenlock forma- 

 tion and were therefore of Upper Silurian age. 



In the further survey of Wales (1855) Aveline discovered proofs 

 that what appeared to be similar ' pale slates ' rise out from below 

 the Wenlock Shales farther south, namely, near Newbridge, 

 west of Builth, and near Llandovery ; at both of these localities 

 they follow conformably upon Upper Llandovery strata. This led 

 Aveline to regard the ' pale slates ' as a distinct formation, and to 

 map them as a practically-continuous band all along the western 

 border of the Wenlock Shales and Denbighshire Grits, from Conway 

 on the north to Llandeilo on the south. He indicated them on the 

 maps by a special tint of colour, and named them ' Tarannon Shales,' 

 from the river-valley of Tarannon in Montgomeryshire where they 

 were well developed. In this district of Tarannon he inferred that 

 they follow unconformably on Lower Llandovery shales and grits. 



Although the band coloured as ' Tarannon Shales ' was thus laid 

 down upon the maps and sections of the Geological Survey in 1855, 

 it was not until 1866, when the 1st edition of liamsay's ' Geology of 

 North Wales ' 5 was published, that any official description of these 

 beds appeared. In that work the characters and relationships 

 of the 'Tarannons' throughout Wales were described, and their 

 thickness was estimated at 1000 to 1500 feet. Their stratigraphical 

 position, as then understood, may be summarized in Ramsay's own 

 words : — 



' In the country near Llandovery (Noeth Grug, etc.) and west of Builth the 

 shales lie directly, and apparently conformably, on the Upper Llandovery 

 Beds. A little further north the Tarannon Beds overlap the Upper Llan- 

 dovery strata, and between that point and the mouth of the Conway in North 



Quart. Journ. Geol. Soc. vol. i (1845) p. 14. 



Phil. Mag. ser. 4, vol. viii (1854) p. 307, footnote. 3 Ibid. loc. cit. 



'The Geology of North Wales ' Mem. Geol. Surv. vol. iii (18G6) p. 4. 

 Ibid. pp. 16, 207-208. 



