644 MISS E. M. R. WOOD ON THE [Nov. 1906, 



29. The Tarannon Series of Tarannon. By Ethel M. K. Wood 

 (Mrs. G. A. Shakespear), D.Sc, University of Birmingham. 

 (Communicated by Prof. C. Lapworth, LL.D., F.R.S., F.G.S. 

 Read February 21st, 1906.) 



[Plates XLVII & XLYIII— Map & Sections.] 



Contents. 



Page 

 I. Introduction 644 



II. Stratigraphical Eelations of the Bocks in the Tarannon District. 653 



(A) Typical Section in the Tarannon Eiver 653 



1. Wenlock Series. 



2. Tarannon Series. 



(B) Confirmatory Sections 664 



1. Northern Area. 



2. Western Area. 



III. General Summary 691 



(A) The Tarannon Sequence. 



(B) Comparisons with the Graptolitic Deposits of other Areas. 



I. INTRODUCTION. 



(A) Historical Review. 



Stretching southward for 70 miles from the shores of the Irish 

 Sea near Conway in North Wales, through Denbighshire and 

 Montgomeryshire into the central parts of Eadnorshire near Llan- 

 bister, there sweeps a continuous band of massive greywackes, 

 flagstones, and shales, known to geologists as the Denbighshire Grits 

 or Flags. This arenaceous formation is from 1000 to 3000 feet in 

 thickness, and is acknowledged on all hands to be of Wenlock age. 



Rising out from beneath the Denbighshire Grits, and inter- 

 vening between them and the older rocks which are at present 

 assigned to the Llandovery and Bala, there occurs a series of rela- 

 tively shaly strata which are distinguished on the Geological-Survey 

 maps by a special tint of purple, and are indicated upon the 

 legends as ' Tarannon Shales.' They are laid down on the maps as 

 forming a narrow belt, seldom exceeding a quarter of a mile in 

 width and entering into all the sinuosities of the outcrop of the 

 lower margin of the Denbighshire Grits. 



Although these so-called ' Tarannon ' Beds were not mapped as 

 constituting a distinct formation until 1855, their peculiar mineral 

 characters — ' fine, smooth, slaty strata, grey or light blue, occasion- 

 ally slightly sandy, and alternating with purple bands ' x — had not 

 escaped the attention of the earlier geological workers in Wales. 

 As early as 1843 Sedgwick and Salter had noticed in the Berwyn 



1 Sir Andrew Earn say, ' Geology of North Wales ' Mem. Geol. Surv. vol. iii 

 (1866) p. 207. 



