Vol. 65.] SUCCESSION AROUND PLYNL1MON AND PONT ERWID. 487 



Fig. 8. — Vertical section 

 through the cyphus 

 zone, liheldol gorge. 



Calcareous 

 nodules 



Graptolitic 

 band 



confirm it by noting the position of rich graptolitic horizons with 

 reference to some ' pattern ' which could be easily recognized. 

 Also the upper layer of nodules which is exposed near the 

 commencement of the water-leat, 14 feet above A, the top of the 

 ' pattern ', is found again 400 yards 

 lower down the gorge occupying the 

 same relative position. Por the 

 guidance of those who may visit this 

 section, the outcrops of the nodule- 

 layers and of the top and bottom of 

 the group of most conspicuous ' pat- 

 terns ' are shown on the enlarged plan 

 of the gorge (PI. XXV). Also the 

 diagram of these ' patterns ', and the 

 position with reference to them of 

 certain rich graptolitic horizons, may- 

 prove of use. It will be observed 

 that the thinner flags between the' 

 thicker flags labelled A-Gr are dif- 

 ferently grouped in each space ; their 

 use in identifying a stratigraphical 

 horizon is, therefore, not unlike the 

 use of the Prauenhofer lines in iden- 

 tifying parts of a solar spectrum. 



The 40 feet of strata included 

 between the upper and lower nodule- 

 layers contain several graptolitic 

 horizons. Two of these may be more 

 especially mentioned : the lower is 

 about 25 feet below the upper no- 

 dules, or a few feet below the rather 

 thick flag G. It is most easily acces- 

 sible in the side of the water-leat, 

 about 50 yards west of the anticline 

 which brings up the lower nodule- 

 layer on the south side of the gorge. 



The rocks dip towards the leat, and 

 large slabs can be easily removed 

 (F. 13). The species collected here 

 include : — Monograptus atavus (v.c.),. 

 M. cyphus (s.s.), M. revolutus and its 

 variety austerus, M. rheidolensis (?) 7 

 M. sandersoni, Orthograptus mutabilis, 

 Glyptograptus sinuatus, Gl. tamaris- 

 cus, Climacograptiis hughesi (c), CI. 

 tornquisti (c). The other horizon is 



OOOC? 



Graptolitic 

 band 



Calcareous 

 nodules 



[Scale : 1 inch = 10 feet,] 



scalaris var. normalis, Ol 



about 6 feet below the upper nodules, or 8 feet above A, the top of 

 the ' patterns ' ; it can be most conveniently examined in the side- 

 of the leat, about 50 yards down the river from the point where a 



