part 3] succESSioisr of the atoxian at cliftojst. 225 



were, however, originally fine oolites, and occasionally (A 75 & 78) 

 the original character is little affected ; as a rule, however, only 

 ' ghosts ' of the oolitic grains remain. Other bands (A 76 & 79) 

 are relatively little dolomitized, and are crowded with foraniinifera. 

 Calcisph(Bra too (A 79 & 86) is very plentiful, in some sections 

 (A 74) forming centres round which oolitic grains have developed. 

 The uppermost beds are much like the succeeding S, Beds, and 

 include calcite-mudstones with ostracods (A 69 & 89) and argilla- 

 ceous limestones with alsi-al nodules containing: JMitcheldeania and 

 Spoiif/iostroma. A band about 20 feet from the base is full of 

 ostracods and Spirorbid annelids. A rock-type which is prevalent 

 in all the calcite-mudstones of C^ and S, is now first met with. 

 Hand-specimens of this rock (A ij5 & 66) are dark-grey calcite- 

 mudstones, sometimes showing signs of contemporaneous breccia- 

 tion. Sections, which as a rule include foraminifera and ostracods 

 (PI. X, fig. 4), are seen to consist of minute, generally rounded 

 patches of structureless calcite-mudstone varying in diameter from 

 about '1 to '5 mm. Patches of finer material may be associated 

 with coarser, or the coarse and fine may alternate in laj'^ers. The 

 rounded patches may be closely packed together or separated by 

 a little matrix. These little patches of calcite-mudstone may have 

 arisen in two possible ways : — 



(1) The ' matrix' or material between them is crystalline calcite, and the 

 isolation of the patches may be due to recrystallization of this ' matrix.' 

 The rock may, in fact, be the exact converse, on a small scale, of the pseudo- 

 breccias (see p. 234), in which the ' fragments ' are recrystallized and the 

 ' matrix ' between them is less affected. This is probably the most satisfactory 

 explanation of the origin of the structure. 



(2) On the other hand, the rock seems to show a close resemblance to a 

 structure described by G. H. Drew ^ from the chalky mud-flats of Florida Keys 

 and the Great Bahia Bank. This he showed to be due to the action of a 

 bacterium {B. calcis) on the calcium-salts present in the sea-water. 

 Prof. E. J. Garwood, also, to whom the structure was shown, was inclined to 

 favour the view that it was due to algal activity, even though it does not 

 show any actual algal tissue. 



A'^ery little is seen of the Canuiia Dolomite on the left bank of 

 the Avon ; but there are several small exposures in the railway- 

 cutting between Quarries 3 & 4 and one by the road. Miss M. B. 

 Chapman's anal3"ses of laminosa dolomite and Caninia Dolomite 

 show the presence of 10*32 and 10*96 per cent, of magnesia 

 respectively, while she found none in the Caninia Oolite. 



(D) The Seminula Beds. (Thickness = about 707 feet ) 



(1) S^. The Lower Seminula Beds. (Thickness = 

 160 feet.) — These rocks form the northern part of the Great 

 Quarry. Their base is marked by the entry of Lithostrotiont 

 corals, a band full of Dipliypliyllum occurring in the last 



' ' On the Precipitation of Calcium Carbonate in the Sea by Marine Bac- 

 teria, & on the Action of Denitrifying Bacteria in Tropical & Temperate Seas ' 

 Carnegie Inst. Washington, No. 182 (1914). I am indebted to Mr. E. E. L. 

 Dixon for drawing my attention to this reference. 



s2 



