part 2] PALEOZOIC eocks of the llangollen district. 199 



following forms from 450 ^^ards north of Fron-Llwyd : — M.grieston- 

 ensis Nicol, M. marri (?) Perner, and J/", priodon Bronn. 



From the data hero brought forward it is clear that the ' Pale 

 Slates ' of Grljn-Ceiriog are undoubtedly of Yalentian age ; yet it is 

 obvious that we have but little precise information as to their 

 zonal position. Their remarkable thinness, coupled with the 

 quartz-veining and disturbance of the Tarannon Slates, especially 

 well seen near the Grlyn slate-quarries, suggests that the strata 

 observed at the outcrop do not represent the true development of 

 the formation. 



Summary of the Yalentian Sequence. 



It is now possible to give a conspectus of the Yalentian formation 

 in the whole district. 



The fullest development is seen in the northern outcrops of 

 Mynydd-Cricor and Cyrn-y-Brain, while, though thinner near 



Table IV. 



a=abseiit; i = in£eiTe(l; p = proved. 



r Monograptus c7'e7ttolatus 



rp ) 3£. qriestonensis 



I a r a u 11 n S n r • 



J 31 . cy^ispus 



L 31. turriculatus 



Thickness in feet < 



Non- sequence. 



fM. sedffwicki 



I 31. convolutus 



I 31. gregarius ... , 



B i r k li i 1 1 i a n <j 3£. cyphus 



I Ortliograptus vesiculosus with ) 

 I 3fesograp)tus modestus ) 

 {^Climacograptus actcminatus 



• Thickness in feet j 



O 



3 



p 

 p 



i 



? 



120 to 

 150 



a 

 ?a 



?a 

 ?a 



80 to 

 120 



^ o 



■S ^ I 



C^ OR 



o 



p 



p? 



p 

 p 



200 to 700 to 

 300 900 



I 

 350? 



1 

 900? 



P 

 P 

 P 

 i 



1200? 



a 

 a 

 a 

 P 

 i 



i 

 500 to 

 600? 



Corwen, the series is even there far more imposing than at Grlyn- 

 Ceiriog. 



Owing to the difficulty of collecting and identifying the fossils, 

 to the monotonous lithology, and to the movements that the beds 

 have undergone, it is not possible to dogmatize concerning the 

 presence or absence of some parts of the series ; but it does appear 

 probable that the thinness noted at Glyn-Ceiriog may be in part 

 due to faulting, though chietiy to attenuation of the sediments. 



Over the greater part of the district evidence seems to exist for 



