158 MR. F. DIXEY AXD PllOf. T. P. 8IBLY OS THE [vol. Ixxiii, 



rapidly along the ' north crop ' of the coalfield.^ Further, ' Mill- 

 stone Grit ' overlies Carboniferous Limestone uneonformably in the 

 Chepstow 2 district, east of the area here described ; and Coal 

 Measures succeed the Lower Carboniferous strata with pronounced 

 unconforinity in the Forest-of-Dean Coalfield,^ lyii^g to the north- 

 east. It is very probable that the unconformity now shown to 

 exist on the eastern fringe of the South Wales coal-basin was 

 due to the earth-movement which produced a great break in the 

 Carboniferous succession of the Forest of Dean. 



No detailed comparison of the variable succession in the district 

 that has been described in this paper M'ith the Avonian of other 

 areas need be made, but attention may be dii-ected to a few points 

 of interest. 



The sequence in the westernmost part of our district, although 

 probably lacking the highest portion of the Avonian, parallels the 

 succession in Gower very closely in other respects.'^ But the 

 remarkable dolomitic sequence in the extreme east of the district 

 here dealt with cannot be matched elsewhere in the South- Western 

 Province : its nearest analogue, to be found in the Forest-of-Dean 

 succession,^ presents considerable differences. 



The interval of Avonian time, K to S^ inclusive, is represented 

 by deposits which survived the inter- Carboniferous denudation 

 everywhere in the district here described. 



K. The slight variation of the deposits of this zone throughout 

 the district accords with the general constancy of facies shown 

 by. the K beds throughout the South- Western Province. But the 

 attenuation of the beds in a north-easterly direction indicates 

 that subsidence was more rapid in the south-Avest than in the 

 north-east. 



Z to Sj. The changes of facies and the attenuation of the strata 

 from south-west to north-east indicate (1) that the area of deposit 

 shallowed, as a rule, in a northerly or north -easterty direction, and 

 (2) that subsidence was more rajoid in the south-west than in the 

 north-east. The form of the outcrop — a single band trending 

 north-north-eastwards — precludes any discrimination, on local 

 evidence, between the significance of northerly or north-easterly 

 directions in this connexion. 



In the interval represented by Z. yC,, and the Laminosa Dolo- 

 mite the north-eastward shallowing Avas probabh" slight, the change 

 of facies amounting to no more than an increase of dolomitization. 



In the interval represented b}' the Oaninia OoHte and C,4- S,, how- 

 ever, standard conditions prevailed almost or quite ^ continuously 



' • The Country around Abergavenny' Mem, Geol. Surv. 1900, p. 19. 



- At Ifton (Monmouthshire) : E. E. L. Dixon, Geol. Mag. dec. 5, vol. vi 

 (1909) p. 515. 



=* T. F. Sibly, Geol. Mag. dec. 5, vol. ix (1912) pp. 417, 420-21. 



■* E. E. L. Dixon & A. Vaughan, Gower paj^er. See especially p. 505 and 

 pp. 532-37 ; and compare table (p. 505) with our table on p. 118. 



•^ T. F. Sibly, Geol. Mag. dec. 5, vol. ix (1912) pp. 418-20. 



^ The thin Modiola phase developed at the base of Co at Miskin has not 

 been detected farther west. 



