538 ME. E. E. L. DIXON AND DE. A. VAUGHAN ON [NOV. I9II, 



District approximated to the conditions of South-Western Gower : 

 for at Longland Bay the base of C 2 is not a Modiola phase, but a 

 group of dolomites representing standard gasteropod- and other 

 limestones. 



(v) In the character of C 2 and S x Eastern Gower definitely 

 links itself with South-Western Gower, and dissociates itself from 

 North- Western Gower. In one respect, namely in an absence of 

 'contemporaneous' dolomite and chert from the last-named district, 

 this difference, between Eastern and South-Western Gower on the 

 one hand and North-Western Gower on the other, persists to the 

 top of D. The difference is marked in the case of dolomitization, 

 because outside North-Western Gower many horizons have been 

 affected by this alteration. As a similar difference between southern 

 and northern outcrops is almost as noticeable in South Pembroke- 

 shire ; and as, further, outcrops to the north of either Gower or 

 South Pembrokeshire are largely if not entirely devoid of ' con- 

 temporaneous ' dolomitization at horizons above C , it is evident that 

 a widespread influence inhibiting such dolomitization — to which 

 alteration the conditions of deposition at many horizons would have 

 been favourable — existed in the coastal waters of the Upper Avoniau 

 sea. In Gower this influence first made itself felt in C 2 ; its nature, 

 however, is unknown. 



(vi) During the interval, S 2 and D v the facies of sedimentation 

 in different parts of the area varied little except in dolomitization, 

 as mentioned above ; at the top of S 2 , lagoon-conditions were but 

 occasionally established in South-Western Gower. 



(vii) Though the greater part of the 'black lias,' D 2 _ 3 , of the 

 Eastern District doubtless corresponds to the same group of the 

 South-Western, some of it, Dr. Vaughan concludes (p. 552, table), 

 has been deposited during the formation of part of the com- 

 paratively pure D 2 limestones which underlie D 2 _ 3 in the latter 

 district. That is, argillaceous limestones and shales in Eastern 

 Gower are the contemporaries of pure limestones in South-Western 

 Gower. D 2 is present in North-Western Gower also, and reappears 

 in force, as a mass of pure limestones, along the North Crop of the 

 coalfield at Mynydd-y-gareg near Kidwelly, 1 12 miles farther north 

 still. The occurrence of the pure limestone facies so far north would 

 point to the existence of a marked embayment in the Avonian 

 coast hereabouts, on the supposition that D 2 1 3 was deposited 

 in waters essentially shallower than those of D„ in its ' pure ' de- 

 velopment. 2 It is more probable that the difference between 

 D 2 _ 3 and D 2 is primarily independent of depth. Eor, north-east of 

 Mynydd-y-gareg, along the North Crop between that place and 

 Garn-bwll (Carmarthenshire), the pure limestone facies of D 2 gives 

 place laterally to dark sandy limestones with chert, 3 that is, to 



1 'The Country around Carmarthen ' Mem. Geol. Surv. 1909, p. 79. 



2 This is the view of Dr. Vaughan, Eep. Brit. Assoc. (Winnipeg) 1909, p. 190. 



3 T. C Cantrill, ' The Country around Carmarthen ' Mem. Geol. Surv. 

 1909, pp. 76-77. He tells me that this facies continues for a great distance east 

 of Garn-bwll. 



