^48 Prof. Ansted on the Zoological condition of Chalk Flints, ^c. 



of calcareous beds, the previous denudation of whose exposed sur- 

 face had formed the lower and middle chalk, would now again be 

 the source of similar deposits, and the sponges would begin to 

 be covered by a calcareous mud, and at the same time the sili- 

 ceous matter would begin to deposit itself upon the organized 

 substance of the sponges. After an interval corresponding to the 

 deposit of the first layer of chalk above the flints I suppose an- 

 other pause to have occurred, and a similar growth of sponge to 

 have taken place and been succeeded by other disturbances, and 

 so that such alternations of tranquillity and volcanic eruption con- 

 tinued till the close of the cretaceous period. 



Without some such cause, I cannot see any reasonable expla- 

 nation of the fact, that while flints and siliceous matter are found 

 abundantly in other beds and under various circumstances, it 

 scarcely ever occurs in layers, except in a certain part of the cre- 

 taceous system, that part not presenting any other difference 

 whatever, either mineral, geological or zoological, with the im- 

 mediately preceding strata : neither can I in any other way ac- 

 count for the fact, that single layers of flint and chert occur in 

 other formations*, but not such alternations of flint and calca- 

 reous matter as we find in the chalk, although the origin of the 

 flint appears to have been the same in all qases. 



And finally, I am not assuming in these alternations and pe- 

 riodical eruptions any extraordinary or improbable agent. Im- 

 mediately subsequent to the cretaceous period, and even during 

 its continuance, we have the most decided proof of the action of 

 disturbing forces on the grandest scale — forces to which we owe 

 the disruption of the chalk in the Wealden district, and the po- 

 sitive and complete denudation of strata several hundred feet thick 

 and many thousand square miles in extent, and disturbances, 

 which in the North of Ireland and elsewhere were accompanied 

 by the eruption of igneous matter to an extent rarely if ever since 

 equalled. 



I am also entitled to assume not only the possibility but the 

 probability of the eruption of hot water containing silica in solu- 

 tion, and it is not impossible that this may have gone on con- 

 stantly and without interruption during the whole period ; while 

 with regard to the existence of other calcareous rocks to whose 

 denudation the chalk is owing, I have only to point to the 

 absence of calcareous rocks overlying the beds in the West of 

 England, and the fact that many oolitic outliers of limestone 

 occur distant from the main formations, but bearing marks of 



* Fn the freestone of Portland, in the mountain limestone of the Mendip 

 Hills, in the oolitic limestone of Pickering in Yorkshire, near Poligny (on 

 the north-west of the Jura mountains), and in the grecnsand of Black 

 Down, &c. — See Geol. Trans. 1st ser. vol. iv. p. 420. 



