352 PEOF. S. H. EEYJSTOLDS AND DE. A. VAUGHAN ON [Aug. I9I I, 



twin-stream, where the best section occurs, the succession is as 

 follows (see also fig. 2, p. 350) : — 



7. Shales. 



6. Crinoidal limestone with polyzoa, similar to bands in K 2 . 



5. Coarsely crystalline limestone with mud-pans. 



4. ' Bryozoa Bed ' (140) — red, highly crystalline and crinoidal limestone : 



about 3 feet. 

 3. Coarsely oolitic and crinoidal limestone (139) : about 5 feet. 

 2. Thick sbale, in places crowded with ostracods. 

 1. Shales alternating with limestone-bands, including one full of small 



Chonetes. A grit-band occurs near the base, and below it is a 



calcareous band (142) containing crinoids, while immediately below 



that is the Old Red Sandstone. 



Further Account of the Lithology of the K i Beds. 



In these are several bands of considerable lithological interest. 

 Band 4, the equivalent of the 'Bryozoa Bed' of the Avon and 

 other sections, is a ferruginous, highly crinoidal limestone (see 

 PI. XXIX, fig. 4). Sections show a considerable number of polyzoa, 

 much iron-stained, and occupying spaces between the crinoidal 

 ossicles. This horizon has not been detected in the western twin- 

 stream. 



Band 3 is the coarsest and most remarkable oolite occurring in 

 the whole section, and is precisely comparable to a band occurring 

 at the same level in the Skrinkle section near Tenby. The oolitic 

 grains, which generally are extremely regular, are in the majority 

 of cases developed, round a fragment of a crinoid-stem (see PI. XXIX, 

 fig. 5). The rock contains a large proportion of iron-oxide. This 

 horizon has not been detected in the western twin-stream. 



Band 2. — The well-bedded, somewhat gritty shales are commonly 

 crowded with ostracods, and are well exposed in the eastern twin- 

 stream. In the western stream ostracods are equally plentiful 

 in (126) a calcareous grit (see PI. XXIX, fig. 6), occurring at rather 

 a lower level. 



Summary of the Lithology. 

 (A) Calcareous Rocks. 



Horny limestone, nearly structureless in thin sections, and 

 concretionary limestone, showing imperfect 'Cotham-Marble' 

 structure, are practically confined to the upper part of S 2 , although 

 horny limestone occurs to a slight extent in S . 



Oolitic structure. — A band of coarse oolite occurs in the 

 upper part of K x ; but, apart from this, no oolite is met with until 

 the top of C x is reached. C„ and S x are very largely oolitic ; there 

 is much oolite at the base of S 2 ; and coarse oolite occurs in the upper 

 part of S 2 . Most of the oolitic grains show no centre, some are 

 developed round quartz-grains, many round foraminifera in the 

 C and S beds, while the very coarse oolite in the K beds has most, 



