Carboniferous Limestone, Avon Gorge. 



499 



Tournaisian. 



a. Modiola Phase. — The passage beds between the Old Eed 

 Sandstone and Carboniferous Limestone. 



CaO 



MgO 



CO2 



Fea O3 + AI2 O3 



Insoluble residue 



Total 



44-34 

 5-23 



43-51 

 1-98 

 5-22 



100-28 



The insoluble residue in various specimens of this zone varies 

 between 3-24 per cent and 11-18 per cent, and consists chiefly of 

 quartz flakes and rounded detrital quartz, felspar (much weathered), 

 and carbonaceous material, mainly bitumen. A few microscopic 

 crystals of zircon, some sponge spicules, and casts of crinoid ossicles 

 preserved in chalcedony are also present. Tlie proportion of insoluble 

 residue is unusually large, as is to be expected in view of the probable 

 proximity of slowly sinking Old Red Sandstone land areas. 



1^0 corals have yet been recorded from these beds, and it thus 

 appears that they do not represent coral reefs. 



K. Cleistopora Zone. 

 Ki. Productus bassus subzone. 



Total 



100-27 



100-05 



The insoluble residue content varied from 2*41 to 4"53 per cent. 

 Analysis A shows the average composition and B that of the 

 s[/ecimen having the greatest proportion of insoluble residue. 



The residue consists of well-rounded detrital quartz, chalcedonic 

 silica, small zircons, much kaolinized felspar, and some tourmaline. 



Ko. Spiriferina octoplicata subzone. 



Total 



100-33 



100-97 



100-26 



A, B, and C give the analyses of the lower, middle, and upper 

 portions of this subzone respectively. 



The percentage of insoluble residue varies between "07 and -21, 

 and is therefore very low. 



Corals are rare throughout the subzone, occurring most abundantly 

 at the top. Brachiopods are plentiful, especially Spiriferina 

 octopUcafa. This cannot be a coral reef from which the coral 



