158 



PEor. 0. T. JOXES ox 



[vol. Ixx^di^ 



the underlying shales contain the characteristic fauna of the M.- 

 sedgwicJci Zone, although the stratigraphical relations are disturbed 

 hj a fault. The time -interval between the 2I.-seclgicicki and 

 Hastrites-maximus Zones is nowhere verv great, if one may judge- 

 by the thickness of sediment, and it is probable that the disturbance 

 produced by the intervening fault is comparatively small. 



The shelly horizons of Girvan can thus be fixed within narrow 

 limits on the graptolitic scale, and by comparison of the shelly 

 facies of other areas with that of Grirvan they can be brought into 

 relation with the same scale. Some assistance in this comparison 

 is afforded by the Yalentian rocks of the Lake District, in which 

 J. E. Marr & H. A. Xicholson ^ have recorded various species of 

 trilobites from the mudstones which divide the graptolitiferous 

 shales. 



V. CoilPAEISOX OF THE HaTEEFOEDWEST SuCCESSIOIS' 

 WITH THAT OF GlETAX. 



The Haverfordwest district, where the shelly facies is more 

 completely represented than anywhere else in Britain, affords the 

 most satisfactory comparison with Girvan. During the recent 

 re-survey of that disti'ict the following classification of the Yalen- 

 tian Series was adopted : — 



( 



<: 

 Hi 



<! 



Millin Stag'e. 



Haver ford 

 Stag-e. 



I 



r Canastou Beds. 



I ITzmaston Beds. 

 /^Grasworks Sandstone. 



Gasworks Mndstone. 



Cartlett Beds. 

 1^ Basement Beds. 



A few miles south of Haverfordwest the Yalentian is representee^, 

 by the Eosemarket Stage, which rests unconformably on the Pre- 

 Cambrian. The facies is totally distinct from that of Haverford- 

 west, and resembles more nearly that of Wooltack and Marloes, or 

 of the May Hill and other districts in the South-West of England. 

 Its relation to the Millin Stage is discussed below. 



A few graptolites have been collected near Haverfordwest, and 

 thev enable certain horizons to be determined apjDroximately by 

 direct reference to the graptolitic scale. A form of 2Ieso(/raptus 

 ■modest us which was obtained near the base of the Cartlett Mud- 

 stones seems to be allied to the variety ^^rrrz-^^/^'s H. Lapworth, 

 which is characteristic of the G]yptogrcrpiuS']yersculptus Zone in 

 Central Wales. The local base of the Yalentian in this district 

 mav, therefore, be assumed to represent approximately the same 

 horizon as the base of the Birkhill in those areas where the grapto-- 

 litic sequence is complete. 



1 Q. J. G. S. vol. xliv (1888) p. 654. 



