)»art2j CAKHnMKKKOl'S IJ.MKSIONK UF S. ^\AI,l:S (((AIJIKJ-U. l')7 

 \l. SUMMAUi AM) CuNLIA SIO.NS. 



Suuiiiiaii/iiig the ivsults of our investigation of the eliange> 

 which afti'ct tlie Carboniferous Limestone Series wlien traee<l 

 nortli-east wards along its outcrop between Bridgend and Kisca, we 

 lind that : 



(1) The tliniinution in tliiekness is due oidy in part to actual 

 attenuation of tlie strata, a factor of greater impoi-tance l)eing the 

 overstep of the Millstone (Jrit across successively lower horizons in 

 the Carboniferous Limestone. 



(n) Unconformable overstep by the Millstone Grit cut.s out the iJibuno- 

 phifllinn beds and the Main Seniiitula Zone (S,), the collective thickness of 

 which in the extreme west of the outcri»p amounts to some r2U0 feet. 



(b) Actual attenuation diminishes the collective thickness of the surviving 

 zones K (Lower Limestone Shales), and Z to S, (Main Limesttme; by some 

 700 feet, from about 1500 feet in the west to 800 feet in the east. 



(2) The Lower Limestone Shaloi maintain their lithological and 

 faunal facies substantially unaltered. 



i'S) The zones of the Main Limestone which ]>ersist throughout 

 (Z to S,, inclusive) undergo great changes in lithological and faunal 

 character. 



(a) In the Taff valley, the secjuence of undolomitized limestones with 

 subordinate dolomites which obtains in the west has given place to an 

 almost unbroken succession of dolomites. This is due to a progressive 

 increase in the vertical extent of contemporaneous dolomitization. The 

 faunas, although largely obliterated, maintain a standard facies. 



(6) East of the Taff valley, the lower beds (Z-C) maintain the character 

 of crystalline dolomites with remnants of the standard fauna. The upper 

 part of the sequence (C-S,), however, becomes a Modiola phase of great 

 thickness, composed essentially of dolomite- mudstones with subordinate 

 calcitic beds. 



Our investigations contirm, therefore, the opinion previously 

 expressed by Mr. E. E. L. Dixon ^ that unconformable overstep by 

 the Millstone Grit takes place in this part of the coalfield margin. 

 The discordance of stratification between the Carboniferous Lime- 

 stone and the Millstone (irit is probably so slight that it would 

 be inap})reciable in sections of the junction of the two formations. 

 None the less, the indisputable overstep of the Millstone Grit 

 across a large part of the Carboniferous Limestone Series admits 

 onlv of the conclusion that the junction is a true plane of uncon- 

 formity, det4.'rmined by a period of uplift and denudaticm dm-ing 

 which the beds of the Carboniferous Limestone were shaved oft" 

 oblirpiely from the north-east or north. Outside the district 

 here descril)ed. several facts suggest that the area of maximiun 

 uj)lift at the time of this earth-movement lay to the north-east, 

 rather than to the north. The attemiation of the Carboniferous 

 Limestone Scries continues northwards along the eastern margin 

 of the coalfield, as far as a |M)int near the north-eastern corner of 

 the basin : west of that point, however, the Series thickens rather 



' Newport Memoir, 2nd ed. p. 20. 



