THE AVONIAN OF THE AVON GORGE 
91 
‘ mottled ’ appearance. The hard portions offer a relatively 
large resistance to weathering, and consequently the surface of 
the rock has become extremely irregular. Possibly the struc- 
ture is, to a limited extent, concretionary, for, occasionally, the 
harder portions exhibit true ‘ shell-structure ’ near their surface ; 
the greater number of these masses, however, seem to be due to 
the patchy accumulation of carbonate of lime, for the corallites 
of a large Lithostrotion-mTsAhim can often be seen to pass from 
one hard portion to another. 
The oolitic limestones which occur at various levels throughout 
the D series can usually be easily distinguished from the oolites 
which occur in lower zones. The oolite grains are larger and 
more scattered, and the rock has usually a red tint. (The deep 
red, coarse oolite which was formerly polished and sold as a 
characteristic Clifton rock was derived from beds in the D series.) 
Seminula-ZouQ (S). 
Upper subzone (Sg). 
Returning to the railroad and resuming our walk 
northward along it, the lofty wall of Seminula-\\mQ- 
stone lies at some little distance to the right of the line. The 
upper beds of the Seminula-Zon^ cannot be easily examined on 
this side of the river ; we may, however, note in passing the 
thin capping of which we have already examined on the 
slope north of the New Zig-zag and, immediately beneath it, a 
band of thinner-bedded rock which constitutes the uppermost 
portion of S2. 
Plate VU. 
The rocks of this band are remarkable for a very peculiar 
structure, and they are here referred to as the ‘ Concretionary 
Beds.’ (This term is used merely to describe the appearance of 
the rock, without definitely suggesting a particular mode of 
origin.) The ‘ Concretionary Beds ’ occur only at the very top 
of the Seminula-Zon^, where, in certain parts of the Bristol Area 
(notably at Sodbury and near Westbury), they form a thick and 
extremely striking band. In the Avon Section, these beds can 
not be examined at the point which we have now reached, but 
