100 
THE AVONIAN OP THE AVON GORGE 
The section ends with 15 feet of thin slabby argillaceous - 
limestones, separated by shale partings. These beds exhibit very 
clearly their shallow water origin, both in their lithic characters 
and in their faunal contents. Several of the thin slabby lime- 
stones have undulating surfaces and exhibit sun cracks. Other 
beds are built up of angular fragments of thin shaley lime- 
stone cemented together in a fine-grained matrix. These beds 
were probably formed by the breaking up of thin limestone 
beds immediately after their deposition, and by the im- 
mediate cementation of the fragments. 
The actual base of the Carboniferous Limestone Series cannot 
be definitely fixed for there is a complete passage down, from 
the calcareous series above, into the marls and sandstones which 
are regarded as constituting the uppermost portion of the Upper 
Old Red Sandstone. 
In the lower cutting the junction is concealed by a wall, 
north of which the marls, sandstones and quartz-breccias of the 
Upper Old Red Sandstone are typically exhibited. 
B. Palaeontological Characters. 
We shall now turn back and study the fossils which are to 
be found at each successive level. 
In this palaeontological survey our attention will be directed 
along two main lines of inquiry 
(1) The possibility of making divisions which shall be based 
" upon the occurrence of certain distinctive fossils Diag- 
nosis ’). 
(2) The registration and study of the important and abundant 
fossils which are associated at each level Fauna ^). 
The Corals and Brachiopods are the only two groups which will 
be dealt with in detail, but attention mil be drawn to the genera 
of other groups wherever they form an important constituent of 
the fauna. 
