200 DE. WILLS AND MR. SMITH ON THE LOWER [vol. Ixxviii, 



the original deposition of all the usually accepted zones of the 

 Tarannon Series. These have a peculiar lithology that in practice, 

 as a rule, suffices to differentiate them from the lower series. The 

 relationship of the Tarannon to the Lower Yalentian or Birkhillian 

 still remains obscure. It is probably in the nature of a non- 

 sequence, for there are no signs of an unconformity in the type of 

 sediment, yet the Monograptus-sedgivichi Zone has nowhere been 

 observed, and the Tarannon slates appear to rest in places on the 

 convohttiis zone (Corwen and parts of Cyrn-y-Brain) and in places 

 on that of M. gregarius (parts of Cyrn-y-Brain) or of M. cyplius 

 (Mynydd-Cricor). 



Table IV on p. 199 illustrates the development of the usually 

 recognized Valentian zones in the various areas dealt with. 



(/>) Salopian Formation — The Denbighshire Series. 



(I) General Observations. 



In the course of the present investigation some 70 square miles 

 of the outcrop of the Wenlock and Ludlow Series have been 

 mapped. The rocks comprise the 'Denbighshire Series' of previous 

 workers. 



The La ngollen- Corwen district has been dealt with by various 

 workers from the days of J. E. Bowman ^ ; but they have, for 

 the greater part, confined their attention to the simple southern 

 portion of the synclinorium. 



In 1895 Mr. P. Lake^ put forward the following classification : — 



Dinas-Bran Beds. 



Slaty Beds, with Monograptus leintivardinensis. 



Gritty Beds. 



Mags, with MonograpUis coloniis, Cardiola interrupta, OrtJioceras 



primsevum, etc. 

 Moel-Fferna Slates, with Monograptus flemingi, M. priodon. 

 Pen-y-glog G-rit. 

 Pen-y-glog Slates, with Monograptus personatus, M. priodon, etc. 



Subject to modifications to which reference is made in the sequel, 

 this classification, together with zonal information contained in the 

 papers by Miss Gr. L. Elles and Dame Ethel Shakespear (nee "Wood), 

 mentioned on p. 177, has formed the basis of our work. 



That the strata of the Denbighshire Series are of shallow-water 

 origin is suggested by the great thickness of rocks of one litho- 

 logical type, and by the presence of internal brecciation, cross- 

 bedding, and contemporaneous erosion-phenomena. 



The series is probably between 4000 and 5000 feet thick, and is 

 composed (with the exception of the Pen-y-glog G-rit) of what 



^ Brit. Assoc. Eep. of Sections (Glasgow) 1840, p. 100 ; ihid. (Plymouth) 

 1841, p. 59 ; and Trans. Manchester Geol. Soc. vol. i (1841) p. 194. 



^ Q. J. G. S. vol. li, pp. 9-22. This paper contains a bibliography complete 

 up to 1895. 



