Thomson. — Genloyii of Middh- Clarence and U re Valleji^. 333 



the redeposition of silica originally scattered throughout the beds in which 

 the flint occurs. The fact that the evenness of the bedding-planes gives 

 place to rolls around the largest flint lenticules seems to prove this for the 

 flint-beds. The structure of the flint-dolomite exteriors is also more in 

 accord with a secondary than with an original deposition. 



Cotton (1918), in examining by the deductive method the kind of 

 deposits to be found in the continental shelf, showed that with advances 

 and retreats of a subsiding shelf corresponding to fluctuations in the ratio 

 of waste-supply to rate of subsidence there may be intercalations of foreset 

 and pelagic beds in the topset beds towards the outer part of the shelf, 

 while farther seaward there will be no intercalations of topset beds but 

 an alternation of pelagic and foreset beds. In this way, he suggests, may 

 perhaps be explained the intercalation of the Amuri limestone between 

 mudstone and marl in Marlborough. Interpreting the chalky limestone 

 as pelagic and the marly limestone as foreset beds passing into pelagic, 

 the frequent alternations of these two kinds of rock in Marlborough receive 

 a satisfactory explanation. Moreover, since both kinds of beds were laid 

 down on the broad continental slopes, and since fluctuations in the supply 

 of waste are regional, it is probable that bands showing such alternations 

 must have a wide geographical range, and can therefore be used in correla- 

 tion over neighbouring areas. In respect to the pelagic beds Cotton's 

 deductions are unaffected by the method of deposition, whether partly 

 chemical or solely by accumulation of tests of marine organisms. 



The following are the reasons for considering that the chalky variety 

 of Amuri limestone is in large part chemically deposited : — 



(1.) Its lithological nature. As Marshall states, by far the greater pait 

 of the rock consists of very fine-grained calcite. with isolated but fairly 

 numerous tests of Glohigerma. If this fine-grained material is derived 

 also from broken tests of Foraminifera, there must have been great com- 

 minution of the tests before consolidation, and it is difficult to imagine 

 any agency which could effect this and yet leave the tests of Glohigerina 

 in such perfect condition. If, on the other hand, the fine-grained material 

 is the result of solution of tests and recrystallization, it is again difficult 

 to see why the tests of Glohigerina have escaped the destruction of form 

 the others have suffered. On the other hand, there is no difliculty in 

 imagining the tests settling gently into a fine mud of chemical origin, and 

 ])reserving their form during the recrystallization that has certainly in some 

 cases taken place. 



(2.) 7^.9 chemical nature. Specimens of chalky limestone from districts 

 where the flint-beds occur and others from districts where flints are rare 

 show ahke a considerable silica-content. The mean of ten analyses I have 

 collated gives 10-25 per cent, of silica. As the rock contains exceedingly 

 little terrigenous sediment, this silica must be mainly ascribed to siliceous 

 organisms if chemical deposition is ruled out ; but neither the remaining 

 skeletons nor the probable casts of such skeletons are present in sufficient 

 amount to account for such a large percentage of silica. The position 

 becomes worse if we add in the silica of the flint-beds, supposing this to be 

 derived by solution and migration of silica from the overlying limestone 

 and not from redeposition in situ of the silica of radiolarian or diatomaceous 

 oozes. 



The presence of dolomite in the flint-beds is difficult to explain if the 

 limestone is a deposit accumulated in deep, cold water solely from skele- 

 tons of marine organisms. The total amount is not very great, and perhaps 



