94 
THE AVONIAN OF THE AVON GOROE 
which have been formed round shells and other organic debris; 
the rock may consequently be broadly described as a pisolite. 
In certain seams of Seminula, the majority of the shells are sur- 
rounded by concretionary crusts and, occasionally, comparatively 
large pieces of coral (such as Syringofora) are seen to be similarly 
included in a concretionary envelope. A great many of the 
‘ fragments ’ are merely broken pieces of concretions and many 
complete concretions occur broken across, with the component 
parts displaced. 
Lastly, it is important to notice that the S2 series in which 
these pisolites occur contains numerous beds which are almost 
entirely built up of continuous coral masses (Lithostrotion and 
Syringopora). 
A probable explanation of the origin of the pisolite appears 
to be the following : — The large coral masses which grew in 
clear shallow water near the shore line impeded the rapid re- 
moval of carbonate of lime, brought down from the land, and 
consequently the percentage of lime was necessarily maintained 
at a high level. Hence the shells and coral debris which accu- 
mulated on the floor were thickly coated with carbonate of lime. 
Many of the concretions were probably broken up almost as 
soon as formed, and the whole accumulation of debris, coated 
and uncoated, broken and unbroken, was rapidly cemented by 
the interstitial deposit of carbonate of lime. 
Lower subzone (SJ. 
All the beds below the ‘ pisolite ’ band, as far as the 
Plate VI 
extreme north end of the quarry, belong to the 
Lower Seminula-ZonQ. 
Thick limestones, mostly dark in colour but of very varying 
texture, build up the greater part of this subzone ; they are 
separated by shales whose thickness increases more and more 
as we approach the end of the quarry. 
One of the most striking of the limestone-typeS is termed 
‘ China 'stone ’ ; this rock has a perfect conchoidal fracture and 
is as compact in texture as a hone ; a fresh fracture is black, 
