Carboniferous Limestone, Avon Gorge. 501 



The insoluble residue resembles that of the Zg subzoDe. 



These beds are approximately contemporaneous with the igneous 

 rocks of the Weston-super-Mare district, and the proximity of this 

 area, combined with current action, may account for the great 

 variation in the amount of insoluble residue between the base of the 

 Z and the beginning of the C zone. 



C. Caninia Zone. 



Ci. (1) Laminosa Dolomite; (2) Caninia Oolite. 



C2. (3) Caninia Dolomite. 



Total . . 100-26 100-14 100-05 



The percentage of insoluble residue is low, being never greater 

 than 3"48 per cent and usually below 1 per cent. In view of the 

 proximity of igneous action the low percentage of insoluble residue is 

 remarkable. 



The residue consists of black carbonaceous material with some 

 bitumen, greyish devitrified glass, brown-stained quartz grains, some 

 minute flakes of quartz, and chalcedonic silica. These beds contain 

 an appreciable amount of phosphate, probably calcium phosphate. 



Total . . 99-88 100-26 



Analyses A and B correspond to the specimens having the greatest 

 and least percentage of insoluble residue. The high percentage may 

 be due to igneous activity in the adjacent district of North Somerset, 

 but the direction of the prevailing currents would have to be 

 materially different from that of the period during which the 

 immediately preceding rocks were deposited, as they contain a very 

 small amount of insoluble residue. jSTo diminution in the percentage 

 of insoluble residue was detected in the marked coral bands. 



The residue consists of well-rounded detrital quartz, often brown- 

 stained, bitumen, other fine carbonaceous material, and siliceous 

 sponge spicules. Quartz flakes occur in the upper part. 



