490 MR. E. E. L. DIXON AND DE. A. VAUGHAN ON [Nov. 191 1, 



dolomite, there are occasional large irregular bodies of dolomite, 

 similar in its coarse grain and its association -with clay to the 

 dolomite characteristic of pseudobreccias, but evidently of some- 

 what different origin. Another peculiar feature of the pseudo- 

 breccias, noted by Mr. Tiddeman and difficult of explanation, is 

 the occurrence of numerous pits, as a rule circular, 18 to 30 

 inches in diameter and a foot deep, but also irregular and larger, 

 distributed evenly over the upper surface of some of the beds 

 (see PI. XXXVIII, fig. 1, taken from an example in the North- 

 Western District) ; these pits are filled with grey, buff- weathering 

 clay or, less frequently, with coarse, light dolomite, and, at some 

 levels at least, underlie thin beds of clay continuous with their 

 infillings. A few cases have been observed of a pseudobreccia 

 passing laterally, in a short distance, into clay with limestone- 

 rubble. 



Subordinate to the pseudobreccias are limestones of ordinary 

 types. In places a coarse-grained oolite marks the base of the 

 zone, and other limestones, dark or light, are intercalated at 

 higher horizons. The zone is, however, further distinguished by 

 the presence of thin coaly layers, also noticed in the first place 

 by Mr. Tiddeman (loc. cit.). Some, though thin and valueless like 

 the others, resemble many true coal-seams in their character 

 and association with underclay, and have probably originated in 

 the same way. Thus, an unusually good exposure at Colts Hill, 

 of one lying at a considerable distance from both the top and the 

 bottom of D 3 , showed in descending order : — 



[Section near thin coal in D r — Southern quarry at Colts Hill.] 



Thickness in feet. 



4. Limestone pseudobreccia, with a standard marine fauna. 



3. Coal smut and finely-laminated, carbonaceous shale; in all 



about , 1 1 



2. Underclay with frequent rootlets and much pyrite ; passing 



upwards into shale (3) 3^ 



1. Limestones, pseudobrecciated or ordinary. The upper surface 

 of the highest is soft, but uneven (with projecting corals), 

 and it is uncertain whether it shows a passage or a non- 

 sequence into (2). 



There is no need to postulate a different origin for the smut- 

 bed at Oxwich Point, though found by Mr. Tiddeman to contain 

 many small gasteropods. 



Fauna. — See pp. 548-49. 



Thickness.— Between 400 and 450 feet (Pwll-du Bay). 



Limits. — The faunal change to L\_ 2 is not accompanied by 

 any noticeable lithological change. 



Typical exposures.— Pwll-du Bay and Head. Mumbles Head 

 (including the Middle Head and the lighthouse islets) :— much of 

 the subzone, the dolomitic pseudobreccias conspicuous. North- 

 north-eastern face of Mumbles Hill : — dolomitic pseudobreccias, in 

 several cases ' pitted.' Oystermouth (Colts Hill southern quarry) : — 



