Vol. 56.] FORMATION AND ITS GRAPTOLITE-FAT7NA. 417 



I have also collected or examined graptolites from other areas of 

 Ludlow rocks in Britain in addition to these three main districts, 

 as for example from the Dee Valley, the Lake District, Dudley, 

 Abberley, etc. In the present paper I propose first to deal with the 

 succession, lithology, and graptolitic fauna of each of the three main 

 districts ; and afterwards to deal briefly with the supplementary 

 districts, and show to what extent the results arrived at accord 

 with those worked out in the more typical areas. The last part of 

 the paper is devoted to the description of the Lower Ludlow 

 graptolites themselves. 



II. Literature. 



In reviewing the history of stratigraphical research I confine 

 myself as much as possible to that in Great Britain, but when 

 considering the purely palaBontological literature on the graptolites 

 of the Lower Ludlow Beds I briefly summarize some of the more 

 important results arrived at abroad. 



(1) Stratigraphical. 



In the year 1839 Murchison first described and defined these 

 beds in the typical area of Ludlow. In his * Silurian System ' he 

 showed that ' they constitute a great argillaceous mass .... of mud- 

 stone .... more argillaceous, less sandy and calcareous ' (pp. 204- 

 207) than the Upper Ludlow rocks. He pointed out that they are 

 rich in fossil organisms, and are limited both at their base and summit 

 by well-marked calcareous beds. He also noted the presence of these 

 rocks in other areas along the Welsh Border, as in the undulating 

 country between the Vale of Radnor and the Wye, and in the 

 Long Mountain, etc. He gave a complete section through the 

 Wenlock and Ludlow Beds in the neighbourhood of Builth, noting 

 especially 'the thin band of impure limestone ' (op. cit. p. 315), made 

 up almost exclusively of the small Terebratula navicula, which he 

 believed to be the representative of the Aymestry Limestone. 



In 1846 appeared the first volume of the Memoirs of H.M. 

 Geological Survey. 1 In this the Survey officers, referring to the 

 Lower Ludlow rocks of the Builth district, grouped the beds in 

 descending order as follows : — 



1 (a) Argillo-arenaceous rocks, with much oblique bedding and other evi- 

 dence of irregular accumulation. Many of the beds are arranged 

 in large irregular concretions. 210 feet. 

 (b) Thin limestone-beds. 10 feet, 

 (e) Same rocks as at (a). 300 feet.' 



These Lower Ludlow Beds are described as being limited at their 

 l>ase by nodules and interrupted beds of limestone (Wenlock Lime- 

 stone), and at their summit by a thin and interrupted band of 

 limestone composed of little else than the remains of Pentamerus 

 Knightii (Aymestry Limestone). 



1 Mem. G-eol. Surv. vol. i : ' Formation of the Eocks of S. Wales & S.W. 

 England ' p. 23. 



