Vol. 67.] THE AVONIAN OF BTJRRINGT0N COMBE. 351 



Cleistopoba Zone (K). 



Extent. — The whole series between the Old lied Sandstone and 

 the commencement of the massive limestone of the Zaphrentis Beds. 

 The beds are exposed in the twin-streams which enter the Combe 

 from the south. In the western stream the K beds commence at 

 about the position of the Groatchurch cave. While the Ji l and lower 

 K 2 beds are fairly exposed, especially in the eastern stream, the 

 upper K 2 beds can scarcely be seen at all. 



Thickness.— 500 feet. 



The K beds show a greater amount of lithological variation than 

 any other part of the section, and are the only part of it in which 

 grit-bands occur, or in which true shales have any appreciable 

 development. 



Upper Cleistopoba Zone (K 2 ). 



Extent. — Erom about the level of the Goatchurch cave in the 

 western twin-stream, or its equivalent position in the eastern, to 

 above the point where the main hillside path crosses the eastern 

 twin-stream. 



In the western twin-stream the highest exposure of the K Q beds 

 is a fine-grained crinoidal limestone (122) just below the Goatchurch 

 cave ; there is a similar exposure at a little waterfall about 30 yards 

 higher up, after which the exposures become very bad. In the 

 eastern twin-stream there are no exposures of the highest beds ; 

 but, at a point where a minute feeder of the stream (shown in the 

 6-inch Ordnance map) enters on the east, a fairly continuous section 

 •commences and extends to the base of K 2 . The rocks consist entirely 

 of narrow bands of dark crinoidal limestone containing many 

 brachiopods, alternating with shales which do not as a rule split 

 w r ell and have often a tendency to conchoidal fracture. In the 

 lower part of the series are several bands of crinoidal limestone 

 crowded with polyzoa and small gastropods (see PI. XXIX, fig. 3), 

 and comparable to bands occurring at the same horizon in the Avon 

 section and at Portishead. This polyzoa-band is also seen in the 

 western stream. 



Lower Cleistopoba Zone (K v including Km). 



Extent. — In both twin-streams the K x beds may roughly be 

 said to occupy the stream-course between the upper and the main 

 paths, commencing at about the level of the main path and passing 

 down into the Old Eed Sandstone at about the level of the upper 

 path. 



The lithological variability stated to be characteristic of the K beds 

 is especially marked in the case of the K x beds. In the eastern 



