THE AVONIAN OP THE AVON GORGE 
101 
Upper ‘ Old Red Sandstone.’ 
The green and red marls and grits, which immediately under- 
lie the lowest beds of the Carboniferous Limestone, are almost 
entirely concealed on the Clifton side of the Avon, but, on the 
Leigh Woods side, these beds are well exposed for examination. 
They have there yielded, in considerable abundance, scales of a 
Rhizodont fish which is, most probably, a species of Strepsodus, 
a genus which has only been previously recorded from rocks of 
Carboniferous Age.^ 
CLEVEDONIAN OR LOWER AVONIAN. 
fK, 
Cleistofora-ZonQ : K = ^ Kj 
Ik^. 
Zonal Index : — Cleistopora cf. geometrica. 
Diagnosis : 
The occurrence of Cleistopora snad the absence of ZapJirentis. 
Characteristic fossils: 
Productus bassus, Eumetria aff. carhonaria, Chonetes cf. 
Buchiana, and Spiriferina cf. octoplicata are all abundant 
within the zone and are rare or absent above. 
Ijower Limit: 
The Modiola-Phase constitutes the basal portion of the zone 
and presents a transitional stage between the conditions pre- 
valent during the deposition of the uppermost Old Red Sand- 
stone and the standard conditions which became established 
in the main portion of the Cleistopora-Zone. 
Upper Limit: 
The fauna of the uppermost beds of the zone only differs 
from that of the succeeding Zaphrentis-Zone in the presence 
of Cleistopora and the absence of Zaphrentis. 
^ I am indebted for this information to the kindness of Dr. Traquair, 
F.R.S., F.G.S., and Mr. E. T. Newton, F.R.S., F.G.S., to whom the specimens 
were submitted for identification. 
