THE AVONIAN OF THE AVON GORGE 
79 
zone of A the fauna of B became established, or how late it 
persisted into that of C. Hence B must be regarded as an 
interruption of the zonal sequence. 
Any such interruption of a zonal sequence, by deposits formed 
under conditions distinct from the standard conditions which 
have been selected for the purpose of zoning, is termed a ‘ Phase.’ 
In the South-Western Province the standard conditions must 
be chosen to be those which, under normal circumstances, existed 
at moderate depth, for these are the conditions which prevailed 
during the greater part of Avonian time throughout the Province. 
Two ‘ phases ’ are co-extensive with the Province ; — the Modiola 
Phase of the Cleistopora-Zone characterized by shallow-water 
conditions, and the S2-Phase of the Seminula-ZowQ, which was 
deposited in shallow water containing an excess of carbonate 
of lime. 
Certain phases are purely local within the Province and the 
most important of these is the dolomite -phase of the Syringo- 
thyris-ZoxiQ which is characterized by abnormal shallow-water 
conditions. Since however this phase is undeveloped in part 
of the area (for example, in the Mendips) the faunal succession 
under standard conditions can be satisfactorily filled in. 
In the Bristol Area, the Brachiopods and Corals are the only 
two classes which are sufficiently abundant throughout the 
sequence to render a system of zones, founded upon their 
faunal variation, both reliable and useful. 
It has been found advisable to study the variations of at 
least two classes, in order that the zones constructed from Coral- 
variation may be checked by the simultaneous Brachiopod- 
variation. For if, at each of two distant points within the 
Bristol Area, two time-scales be graduated in terms of the varia- 
tion-incidents which are exhibited by the Corals and Brachiopods 
separately, it is found that the same variation-incident, which 
is marked on both Brachiopod-scales, does not occupy exactly 
the same position with reference to the graduations of the two 
coral scales at the two localities. (In fact, in passing from south 
