134 MR. F. DIXEY AXD PEOP\ T. F. SIBLT OX THE [vol. Ixxiii, 



t'(jntinuous with the outcrop of Limestone on the north, but 

 faulted against Lower Limestone Shales on the east. The fault, 

 running south-westwards past Pen-evrn to Pen-v-lan, cuts out 

 almost the whole of the shales with thin limestones at Pen-cjTu. 

 This would account for the apparent greatly-reduced thickness of 

 the shale group (estimated from width of outcrop) at the latter 

 place, mentioned in the Geological Survey memoir.^ 



About 650 yards east of Llansannor Church, black shales and 

 thinly-bedded argillaceous limestones are exposed in a small 

 excavation and in the lane adjacent to it (Glam. 41 SE A 7). 

 The limestones, often ostracodal, contain many brachiopods. 

 including JProihictus aff. hciHsus and Sj)iriferiiia octopJicata. The 

 shales yield small J/o^Z/o/rz-like lamellibranelis. 



(B) Main Limestone. 



The sequence of zones is most nearly complete in the Avest of 

 the district, and there it can be recognized in detail. The area 

 west of the river Taff will, therefore, be descrilied first. 



West of the Taff. 

 Za pit re lit is Zone and Lower Caninia Zone : Z tt C^. 



Tlie sequence Z-C. is represented by one colour on our ma]) 

 (PI. XY) because evidence for a satisfactory delimitation of Z and 

 C\ is not forthcoming. 



In the westernmost ])art of tlie outcrop the succession is mainly 

 one of undolomitized crinoidal limestones and oolites, contempora- 

 neous dolomitization being practically confined to two horizons, 

 namely, a lower level, in Z, and an upjjer level (the Lcii/iinosa 

 Dolomite) in C^. Eastwards, dolomitization affects a steadily- 

 increasing portion of the sequence until, in the Taff valley, it has 

 produced an unbroken succession of dolomites. 



The folloAA-ing table summarizes the secpience in the outcrop 

 between Miskin and Grroes-faen. where the l3eds are, on the whole, 

 best exposed. Xo precise limits can be assigned to the several 

 subdivisions : and the thicknesses stated, estimated in most cases 

 from dip and outcrop, are only i-ough a])proximations : — 



Z & C\ between Miskin and Groes-faen. 



Feet. 

 f 8. Caainia Oolite : pale-grej' oolite, thicklj'-bedded to massive. "| 

 ; 7. Laminosa Dolomite: finely-crj-stalline dolomite, crinoidal > 340 

 ^ 1 in part. J 



^ *^ 6. Ci'inoidal limestones, dolomitic at the base. A little chert, i -ioa 

 I Abmadant corals and brachiopods (■}, C,). j 



Oolite : crinoidal. cuiTent-bedded 20 



I 0. Dolomites and dolomitic limestones, ci'inoidal in part. AK^,.^ 

 I little chert near the base. J "" 



Z-{ 4. Crinoidal limestones. Avith argillaceous layers, chert, and 1 

 ! beekite. Zaphrentis flelanouei and abimdant brachiopods > 35 



L (z,).J 



^ Brideend Memoir, p. 11. 



