256 Geological Society. 
is confirmed, the Cotham-Marble-like layers consisting of Mitchel- 
deania or Spongiostroma, the two forms being commonly asso- 
ciated. Mitcheldeania is the most persistent of the calcareous 
alge, ranging from the extreme base of the section to the top 
of S,. Ortonella is characteristic of the K Beds; Solenopora is 
also found in those beds. Spongiostroma, often associated with 
Mitcheldeania, is the prevalent organism in many ot the calcite- 
mudstones of C, and S. Girvanella has been met with in D, and 
Aphralysia in C, and 8. The ‘ Seminula-pisolite’ structure of 
Vaughan proves to be algal in origin. 
Dolomitization is considerably more widespread in the Avon 
section than had previously been ascertained. There has been a 
little dolomitization at one or two levels in the K Beds, but it is 
not a characteristic feature of that horizon. The matrix of the 
crinoidal limestone which forms practically all the Z Beds is nearly 
everywhere dolomitized. The almost complete dolomitization of 
the Lower C, Zone (Laminosa Dolomite) and of C, (Canina 
Dolomite) has long been familiar ; but it has scarcely been recog- 
nized how widespread is dolomitization in the calcite-mudstones 
of S, and in the Lithostrotion Limestones of S, and §,. Oolites 
tend to resist dolomitization. Mr. Dixon’s conclusions regarding 
the Gower dolomites seem to be perfectly applicable to those of the 
Avon section; everything indicates that the main dolomites are 
‘ contemporaneous’. Pseudobrecciation, as described from Gower 
by Mr. Dixon, is most characteristic of the foraminiferal limestones 
of the D Beds; but a novel feature is the recognition of an 
analogous recrystallization in sandy limestone from both D, and 
the Bryozoa Bed (K,). It is suggested that a peculiar variety of 
calcite-mudstone may be due to recrystallization on a microscopic 
scale somewhat analogous to that of the pseudobreccias on a larger 
scale, except that in the calcite-mudstones it is the ‘ matrix’ (not 
the ‘ fragments”) which is recrystallized. The ‘rubbly’ limestones 
so characteristic of the D Beds are also probably related, and owe 
their origin to a concretionary and recrystallization process by 
which the lime gathered in the nodules, from which the iron and 
shaly material became separated. 
Miss Chapman having published several chemical analyses, and 
Mr. E. B. Wethered having studied a number of insoluble residues, 
no further work on these lines was undertaken. The Avon 
succession is shown in a detailed vertical section appended to the 
paper. 
st 
