part 2] PALEOZOIC hocks or the llae^&ollen district. 189 



Table II. — Approximate Thicknesses in feet of the Formations prom 

 THE Brtn Beds to the Cokwen Grit, calculated from their Outcrops. 





Peii-Creigiau Barcut 

 (western edge of 

 Sheet 121). 



.2 5 





03 



5^ 







d 

 >> 



f Upper or 

 Glyn massive grit 



20 



20? 



20 



10 



10-0 

 100- 



a 



a 



a i 



C r w e n I J 



Grit. ^^^TJ' «^- 

 platy grit 



and Glyn 



1^ Limestone . 



U 



"1 



50 



50 + 



20 



20 



20 



Dolhir Beds {sensu 

 stricto). 



j>1860 



^1500 



J 



1020 



1 

 >1050 



^1000 



970 



840 



620 



Ty'n-y-twmpath Beds 



300 



180 



150 



^ U 



J u 



U 



U 



U 



Blaen-y-cwni Beds 



120 



90 



100 



50 



a 



a 



a 



a 



Pen-y-graig Ash 



a 



10 



20 



4 



10 



360 



a 



a? 



a? 



Brvn Beds 



120 



150 



210 



210 



400 



500 



560 



! 









[U=unrecognizable ; a=absent. See also diagrammatic section, fig. 3, p. 185.] 



Ty'n-y-twmpath Beds preclude us from accepting this wide range 

 of the fault westwards, unless it passes to a lower horizon and cuts 

 out the lower part of the Bryn Beds in this region, instead of the 

 top, as suggested by Groom & Lake. The rapid westward thinning 

 of tlie Bryn Beds might thus be accounted for ; but there is no 

 definite evidence for such a fault. 



From Gelli eastwards, however, we have drawn a fault similar 

 to that adopted by Groom & Lake, separating the Bryn Beds 

 from the Dolhir Beds proper, and cutting out the Blaen-y-cwm 

 and Ty'n-y-twmpath Beds ; but in our view the Pen-y-graig Ash 

 is probably absent by non-deposition east of the Glyn Valley, and 

 most of the increase in thickness of the Bryn Beds is due to 

 lithological changes affecting especially their lower part. In order 

 to produce the arrangement of the outcrops observed, the fault 

 must be inclined at a lower angle than tlie beds, and is probably 

 a thrust. 



The only other feasible explanation of the observed facts would 

 involve an unconformity with overlap, or at least a marked non- 

 sequence between the Ashgillian and the Caradocian beds east of 

 Gelli. There is nothing, however, in the nature of the sediments 

 composing the Ashgillian to suggest an unconformity. The gap 

 in the sequence, also, is too extensive to be readil}'' explained by a 

 non-sequence. 



