Yol. 56.] FORMATION AND ITS GRAPTOLITE-jFAIJNA. 423 



that the Lower Ludlow Beds are capable of being grouped into at 

 least four graptolitic zones, but these are apparently less persistent 

 geographically than are the zones worked out in other Silurian 

 formations. Indeed, speaking generally, the Lower Ludlow grapto- 

 lites (with some notable exceptions which are of well nigh world- 

 wide distribution) have for the most part an extended vertical 

 range and a limited geographical distribution. When, 

 however, we consider the rapid rate of accumulation of the Lower 

 Ludlow strata as compared with that of the Birkhill Shales, for 

 example ; and when we remember that the graptolites as a family 

 were dying out at this period, the somewhat imperfect nature 

 of the zonal divisions of the Lower Ludlow Beds is hardly a 

 matter for surprise. Again, owing to the fact that graptolites are 

 of extremely rare occurrence (in Britain) in limestones, and indeed 

 in some cases for a certain distance both above and below them, 

 the graptolitic succession is naturally more incomplete in those 

 areas where the more calcareous development of the Lower Ludlow 

 exists than it is where the purely argillaceous mudstone-facies 

 occurs. 



If we consider the evidence derived from all the districts 

 examined, it may be said that the Lower Ludlow Beds (including 

 the Aymestry Limestone) are, broadly speaking, divisible into five 

 main zones, as follows : — 



(5) Zone of Monograptus leintwardinensis, Hopk. 



(4) „ M. tumescens, sp. nov. 



(3) „ M. scanicus, Tullb. 



(2) „ M. Nilssoni (Barr.). 



(1) „ M. vulgaris, sp, nov. (at the base). 



These zones are not all equally well developed, or even present 

 in every area, but their limitations will be considered when each 

 district is described in detail. 



IY. Description of Typical Districts. 



(A) The Ludlow District. 

 (1) Physical Features and Structure. 



The typical district of Ludlow is so well known to geologists that 

 only a very brief notice of its structure and physical features will be 

 necessary. All along the Welsh Borderland the various beds of the 

 Wenlock and Ludlow formations strike in a general north-north- 

 easterly and south-south-westerly direction ; but near Downton, 

 some 5 or 6 miles west of Ludlow, the direction of strike changes 

 abruptly to almost due west and east, owing to the fact that the 

 beds are folded into an anticline, the axis of which runs north-east 

 and south-west. On the Upper Ludlow Beds at the extreme 

 north-eastern end of this anticlinal fold is situated the town of 

 Ludlow. 



