THE AVONIAN OF THE AVON OORGE 
93 
and charged with abundant carbonate of lime, and it seems a 
fair deduction from the remarkable character of the rock that 
the occurrence of these beds marks a datum level of absolute 
time, almost comparable in exactitude with that marked by a 
continuous lava sheet. 
We will now proceed to the ‘ Great Quarry,’ ^ where Si and 
the lower part of S2 are splendidly exposed. 
The highest beds, which come down to the floor of the quarry 
at its southern end, consist entirely of a fine white oolite, the 
‘ iSemmw/a- Oolite.’ This Oolite is seen as a conspicuous thick 
band in the face of the Quarry and, like the similar band lower 
down in the sequence, is characterized by the vertical joints which 
traverse its whole thickness, and by the weak development of 
bedding planes. (These characters are, however, much more 
'striking in the Caninia-OolitQ lower down and we shall con- 
sequently defer, for the present, an inquiry into their origin.) 
. The base of the Upper Seminula-ZonQ in the Great Quarry is 
marked by a good development of a very peculiar type of lime- 
stone which is here referred to as ‘ pisolite.’ 
This rock recurs at frequent intervals throughout the S2 series 
and is, in the South-Western Province, confined to that subzone. 
It can be studied in the Avon Section at various levels, but 
nowhere is it so strikingly displayed as in the basal beds of the 
S2 subzone, in the middle of the Great Quarry. (In the repeti- 
tion of the S2 series, south of the Fault, the ‘ pisolite ’ is well 
shown, at a higher level, a few yards south of the Suspension 
Bridge.) The rock has the aspect of a conglomerate or breccia 
in which the ‘ fragments ’ are of white limestone, cemented in a 
limestone matrix. When the ‘ fragments ’ are examined they 
are seen to have, in all cases, a concretionary structure, although 
this structure may only be evident in the outermost layers. 
The greater number of the ‘ fragments ’ are large concretions 
^ This quarry is now used as a Rifle Range by the Clifton College 
V olunteers. 
