]»art2] CAKHOMFEHULS 1,1 M KNT«)>' K OF S. \\ A I.KS COA l.ll K l.l>. \ '>'.^ 



Tlic upper beds of the (luurry are grey frinoidal (lolomitf^. lii{;lily 

 <»<)litic in Homo beds, and contiiining Srlifllu'ienelln cf. rrenixtrin and Siiirijcr 

 doihratux. 



In the Rhymney valley, a cutting'- on the Brecon &. Merthyr liailway 

 (Mon. 27 S?i .\ 1 ). iinniedialely below Machen (.iuarrj*. exposes the base of 

 the Mam Limestone. The section shows 8 or 10 feet of thijdy-b»'dde<l. dolo- 

 ujitic limestones, with some highly argillaceous ban<lH and an abundant 

 development of crinoidal chert, underlying' yrey dolomites without chert. 

 The change from the argillaceous, cherty beds to the normal grey (hdoiuitc 

 is sharj). ■ The lowest bed exposed is a V>ajid of dark-grey crinoidal limestone, 

 islightly cherty but ncm-dolomitic, and containing Choneteis cf. fninlrfiiKin and 

 ChoneteK crnnttistria Vaughan (non M"Coy)in profusirm, together with Spirift-r 

 cUithrafitg and Srliflhrieitella cf. m'nii<iriii. 



The grey d(»lomites of this railway-cutting and Machen Quarry !<how none 

 of the special feattires of the beds in L'efn-garw Quarry, described above. 

 In Dan-y-Graig C2"!irry. Kisca, the base of the Main Limotone, again 

 exi)08ed. shows tine-grained dolomites alternating with hard dolomitic shales, 

 and the beds contain only some scanty crinoid-d«'bris. There is evi<lently 

 mnch variation in the character of the lowest beds of the Main Limestone 

 between the TafF and the Kbl)w. 



(iij The JIo(h'()/(f phases, etc., of C,-S,. 



The sequence clearly detined on Cet'n-On, coniprisinjjf two 

 MinUola phases with an intervening band of crystalline dolomites, 

 can be traced for about 2 miles along the outcrop in each 

 direction, west-south-westwards to the neighbourhood of C'efn- 

 carnau-fawr, .md east-north-eastwards to the vicinity of IVn-how. 

 Along this ])ortion of the outcroj) a colour-boundary on our ma)» 

 ( 1*1. XVI ) re))r».'sents the base of the dolomite-band, as e\])lained 

 on }). 141). Farther west, the Modiola phases cease to be traceable. 

 Farther east, the dolomite-band has not been recognized : and, 

 by the time the Jibynjney valley is reached, the wlmle division 

 has probably assmned the essential character of a ^ladlnhi jihasc. 

 which it i)resents in the Kl)bw valley. 



On the eastern side of the Hhynmey valley, in the ravine below 

 Ffwrwm, the Millstone ( J rit oversteps the base of S,, and comes t<» 

 rest directly u}>on the C-S^ beds. This sharj) transgression, which 

 is as.Hociated with the development of a band of (juartz-conglo- 

 memte at the base of the Millstone (Jrit, continues as far as tin* 

 €i-est of the ridge l)etween the Khymney and the Kbbw, and so 

 cuts out a considerable thickness of (' S^ beds. On the desct-nt to 

 the Kbbw Uiver, however, the <juarty.-conglomerate dies (Uit, the 

 base of the Millstone (Jrit retrogresses, an<l the u]>per beds of C-S* 

 emerge almost comj»letely. 



The lower Moditthi phase ((', ^'j i« l>y far the thickest 

 group, and attains a thickness of some 2o() feet on (V-fn-On. It 

 is con»i)osed largely of dolomite-mudstones, very varied in colour, 

 with thin partings and lentieles of shale an<l clay, ami a consider- 

 able intercalation of very line-grained, crystalline dolomites. in 

 the mi«ldle and u])|K'r portions, compact grey linu'stones, nmstly 

 ealcite-mudstones with conchoidal fracture, may be developed in 

 large or small amount. Xo fossils havi' been o])servcd. 



