252 STRATIGEAPHIGAL GEOLOGY 



It should be stated that Dr. Vaughan regards the Cyathaxonia 

 Beds and their equivalents as a sub-zone of the Dibunophyllum zone, 

 but as these equivalents include the Yoredale Beds of Yorkshire 

 and a great part of the " Pendleside Series " of Dr. Hind, which 

 possesses a peculiar fauna of its own, it seems desirable to give 

 special prominence to them in a general scheme, though they are 

 poorly represented near Bristol. 



For convenience of description the more or less different facies 

 which the Avonian Series presents in different parts of the British 

 Isles will have to be treated under seven heads, viz. (1) Bristol 

 and South Wales, (2) North Wales and Derbyshire, (3) South 

 Lancashire and Yorkshire, (4) North Lancashire and Westmoreland, 

 (5) Northumberland and Berwick, (6) Scotland, (7) Ireland. 



1. Bristol and South Wales 



The zonal study of the limestone series of the Bristol district 

 by Dr. Vaughan 1 has made it the typical exposure of that series in 

 Britain. The succession is there complete from the base to the top 

 of the Dibunophyllum zone (D ), but no calcareous representative of 

 the Cyathaxonia zone (D 3 and D y ) occurs in the Mendip or Bristol 

 districts, and its equivalent must be sought in the sandstones which 

 form the lower part of the so-called " Millstone grit." 



The actual thickness of the Limestone Series varies greatly in 

 different parts of the area ; it is thickest in the Mendip Hills, 

 rather less at Clifton, and much thinner to the northward. Litho- 

 logically the series has been divided into four parts, which, with 

 their varying thicknesses, are given below : 



Mendips. Clifton. Wickwar. 



Upper limestones and shales .... 1200 1200 830 

 Middle shales and oolitic limestone . . . 250 300) 

 Lower limestones (encrinital) .... 1200 500/ 

 Lower shales . 450 400 320 



3000 2400 1670 



Dolomitisation is prevalent in the middle part of the series, and 

 affects a greater and greater portion as the beds are followed north- 

 ward. The lithology of the strata is, however, only of local 

 importance, while the zonal divisions established by Dr. Vaughan 

 form a standard by means of which the limestones and shales of 

 other areas, whether they are in similar or dissimilar sequence, can 

 be compared with the Avon section. 



The Cleistopora zone is coextensive with the lower shales, and 

 is divisible into two portions, the lower 100 feet forming the phase 



