VERTICAL CHALK-STRATA. 201 



pasteboard, to two or three inches. These sili- 

 ceous veins often consist of two plates with 

 loose calcareous detritus between them, as if the 

 flint had flowed down or oozed from the sides 

 of the fissures ; and this detritus is frequently 

 composed of shells of microscopic foraminifera. 

 In other cases the cavities are filled with marly 

 and soft chalk ; and in many instances they are 

 empty, but their sides are lined with crystallized 

 carbonate of lime. These fissures are evidently 

 referable to a period long antecedent to the 

 elevatory movements which broke up the chalk 

 strata, and hurled them into their present posi- 

 tion, for they traverse the horizontal layers ; 

 and the subsequent introduction of flint, proves 

 that they were the effect of submarine movements, 

 which took place while the physical condition of 

 the bed of the cretaceous ocean remained un- 

 changed. This fact also corroborates the inference, 

 that the eruptions of steam or vapour charged with 

 silex into the chalk, occurred periodically, and 

 sometimes at long intervals. 



But there are numerous chasms and fissures 

 in the strata which clearly belong to a very dif- 

 ferent epoch ; to a period geologically recent, 

 when the now upper surface of the chalk 

 was rent by earthquakes, and the mountain 



