Vol. 67.] THE AVONIA.X OF BURRINGTON COMBE. 387 



It is worthy of note that the thickness of K traced northwards 

 from Burrington does not diminish as it does in later zones, on 

 account doubtless of different configuration of the shore-line. 



/3 (base of Z). 



Standard conditions (encrinital limestones and shales) became 

 continuous over the whole Bristol-Dinant Province, and the 

 ZajJirentis fauna was established. 



In the uppermost Pilton Beds of Xorth Devon, the fauna is 

 essentially a /3 fauna with a few persistent Devonian forms. 



This is the period of greatest extent of the Bristol-Dinant Province, 

 and has been selected as the datum-line for the vertical sections 

 in fig. 12 (p. 386). 



It is the base-line of the Belgian Carboniferous and of the Tour- 

 naisian of the Belgian writers (an unfortunate fact, since we cannot 

 therefore in strictness include the K zone in the Tournaisian). 



Standard conditions prevailed generally over the Bristol-Dinant 

 Province, and there is a noticeable diminution of thickness both 

 towards the north from Burrington and Dinant, and towards 

 the east from the South-Western Province to that of Belgium. 

 Furthermore, Xorth Devon emerged, and formed the southern 

 boundary of the Bristol-Dinant Bay. The variation of thickness 

 clearly points to a northern and eastern shore. 



[The disproportionate thickening of Z at Burrington and Waul- 

 soit is caused by the oncoming of C ; conditions (see below) at the 

 top of Z.] 



Dolomite is developed towards the northern and eastern shore- 

 lines (for instance, nearly the whole of Z is dolomitized in the 

 Chepstow area north of Burrington, and in the Namur area north 

 of Dinant). 



c,. 



Indications of a shallowing sea are universal throughout the 

 Bristol-Dinant Province, and this is the period of greatest diversity 

 of deposit. 



Two types of deposit dominate the sediments, namely : — 



(1) The eoarsely-crmoidal or ' Petit-Granit ' type, with the knoll or 



Waulsortian type as a variant ; 



(2) Dolomite. 



At Burrington there is a great thickness (625 feet) of the Petit- 

 Granit type followed by dolomite (375 feet). 



Unfortunately, only the lowest and highest parts of the Petit- 

 Granit division are well exposed. At Waterlip, however (south of 

 Burrington and on the southern flank of the ^lendips), where the 

 development is practically identical in thickness with that at Bur- 

 rington and agrees exactly where the two series can be compared, 



