CHAP. X.] CRETACEOUS PERIOD. 175 



and into shallow-water beds, with a minimum of calcareous 

 matter, on the other. 



The Totternhoe Stone seems to indicate a temporary 

 increase in the strength of the prevalent currents, enabling 

 them to carry coarser particles to a greater distance. This 

 bed attains its maximum thickness in the counties of 

 Bedford and Cambridge, thinning out to the north and 

 south. 



Upward through the Lower Chalk there is the same 

 transition to a more purely calcareous deposit that we 

 found taking place horizontally in the case of the Chalk 

 Marl. Deposition was now more uniform, the thickness 

 of the beds above the Totternhoe Stone varying less than 

 those below. At the summit of the Lower Chalk, how- 

 ever, there is evidence of another physical change in the 

 occurrence of one or two layers of grey shaly marl. This 

 horizon is continuous from the Isle of Wight to Suffolk ; it 

 is obscure in Norfolk, but occurs again throughout Lin- 

 colnshire and Yorkshire. There is nothing more remark- 

 able in the stratigraphy of the Chalk than the wide 

 extension of these shaly marls, but they contain little 

 that throws light on the circumstances which led to their 

 formation. 



We might suppose that the shale was due to elevation 

 of the sea-floor and a consequent shallowing of the water, 

 but if this were so we should expect the incoming of a 

 shallow- water fauna, whereas the fauna of the marls is a 

 very small assemblage, and is similar to that of the Chalk 

 below. It seems more probable that the sea was at this 

 time invaded by a flow of cold water from the north in a 

 broad and steady current, which was strong enough to 

 prevent the deposition of much chalky matter, but too far 

 from land to carry much sediment with it. This hypo- 

 thesis of a northern under-current will also account for the 

 sudden disappearance of the creatures which lived in the 



