Chalk and Flint Formation. 3 



expressed his rejection of the leading theory of 

 Professor Jones, 4 and thinks " the direct infiltration 

 (of silica) into susceptible cavities formed by organic 

 decay " the more simple explanation ; and this he 

 supposes to take place " under certain conditions 

 which (however) he does not venture to suggest." 

 Now in considerable districts the chalk is found 

 ivithout flints ; and there, Mr. Mortimer says, " this 

 non-aggregation of silica in the flintless chalk is not 

 due to the want of forms, the result of the decay of 

 animal and vegetable substances, as sections will 

 disclose shapes resembling every form of flint, even 

 banded specimens. These forms, like the flints, are 

 due to casts from cavities left by the destruction of 

 organic substances." But since the chalk itself might 

 thus fill those supposed vacant cavities, and take the 

 cast of those forms, and has done so, as stated by 

 Mr. Mortimer, the question arises, Why did it do so 

 in some large tracts, but leave the cavities to be 

 filled with the aggregation of silica in other large 

 tracts, notwithstanding an equal, if not greater 

 dissemination of siliceous particles (as he alleges) in 

 the chalk of the flintless tracts ? Of this there is 

 no explanation given. There is, therefore, still 

 good reason for a remark quoted by Professor Jones 



4 Mr. Mortimer's objections, which had presented themselves 

 independently to me also on reading Professor Jones's paper, are 

 stated by him as follows : " Since writing the above, the author 

 has read Professor Rupert Jones's paper on ' Quartz, Chalcedony, 

 Agate, Flint, Chert, Jasper, and other forms of Silica geologically 

 considered.' The leading theory propounded he has difficulty in 

 "believing, and considers it should be shown how amorphous lime 

 can be replaced by silica, while calcite and arr.igonite are not. 

 Also that the whereabouts of the displaced chalk should be 

 accounted for. In the Speeton-section of 350 feet, the flint is 

 equal to twenty-four feet vertical." Ibid. p. 354. 



B 2 



