part 2] CAKEOXIFEKOUS LIMESTONE OF S. WALES COALFIELD. 159 



in tlie south-west of the district here described, while lagoon 

 conditions prevailed without sensible interruption in the north-east. 

 Tlie thick, dolomitic Modlola phase of C\-S^ which is developed 

 in the easternmost part of the district agrees, in all its essential 

 features, with the Modlola phase of C,^-S, which succeeds the 

 (Jcuiinia Oolite in the Avon, Sodbury, and Tytherington sections 

 of the Bristol area ^ to the south-east and east. In the district 

 here described, however, the phase, commencing in C,, probably 

 covers the period of the Can in la Oolite. 



EXPLANATION OF PLATES XII-XVI. 



Plate XII. 



Fig. 1. Craig Llanishen, Cefn-On, and the intervening strike- 

 valley: view looking westwards. The featxu"es of the middle 

 distance, etc., reading from left to right, that is, from south to 

 north, are as follows : — ((f) the wooded crest and dip-slope of 

 Craig Llanishen — the quartz- conglomerate and sandstones of the 

 Upper Old Eed Sandstone ; (b) a grassy belt- the highest beds 

 of the Old Red Sandstone and the limestones and shales (1) of 

 the Lower Limestone Shales ; (c) a wooded ridge — the limestones 



(2) of the Lower Limestone Shales ; (cZ) a grassy belt — the shales 



(3) of the Lower Limestone Shales ; and (e) the wooded scarp and 

 crest of the western end of Cefn-On — the Main Limestone. See 

 map (PL XVI).— T. F. Sibly, photo. 



2. The scarp-face of Cefn-On at the head of Cwm Draethen: 



view looking eastwards. The features of the middle distance and 

 foreground, reading from left to right, that is, from north to south, 

 are as follows : — {a) the steep, wooded scarp of the Main Limestone ; 

 (6) a grassy flat, made by the shales (3) of the Lower Limestone 

 Shales ; (c) a low, wooded escarpment, made by limestones in the 

 Lower Limestone Shales ; {d) the ravine of the Draethen brook, 

 cut in the topmost beds of the Old Red Sandstone ; and (e) the 

 dip-sloj)e of Craig Lysfaen — Upper Old Red Sandstone. The 

 features (a), (6), and (c) are displaced in this locality by a dip- 

 fault, the track of which enters the picture at the lower right-hand 

 corner and runs up a ravine in the scarp-face of the Main 

 Limestone. In the distance, the Main Limestone escarpment and 

 the high ridge of Craig Ruperra (Old Red Sandstone) appear 

 faintly. See map (PL XVI), and section (fig. 4, p. 120). — 

 T. F. Sibly, photo. 



3. Machen Quarry, etc. : view looking north-eastwards across the 



Rhymney valley. The crystalline dolomites forming the lower 

 part of the Main Limestone, in which the quarry is opened, 

 determine a bold wooded ridge. The scarp -face of this ridge 

 falls southwards to (1) a grassy flat, succeeded by (2) a low, 

 wooded ridge, the two features so often caused by the shales and 

 limestones respectively, of the Lower Limestone Shales (compare 

 flgs. 1 & 2). The dip-slope of the ridge falls northwards to a 

 cultivated tract which is made by the C^-S^ Modiola phase of 

 the Main Limestone. The Pennant-Grit scarp of Mynydd Machen 

 enters the left-hand side of the picture. See map (PI. XVI). — 

 T. F. Sibly, photo. 



1 A. Vaughan, Q. J. G. S. voL Ixi (1905) pp. 193-95 (Avon), pp. 207-209 

 (Sodbury), pp. 221-22 (Tytherington). 



