4 The Origin of the 



in speaking of Dr. Bowerbank : " He would not 

 pretend that the subject of silicification was at all 

 mastered yet." 



The conclusions which had been formed as to the 

 chalk may be equally premature. Deep-sea sound- 

 ings and dredgings have, in many instances, brought 

 up chalk with microscopic shells and infusoria ; but 

 this may not prove more than that the chalk 

 existing there had been made use of by these minute 

 creatures as furnishing the material of their shells ; 

 just as hens take lime to furnish the material of their 

 egg-shells, and, if they cannot obtain any lime, 

 produce their eggs without shells. It no more 

 follows that the microscopic marine anirnalculae 

 created the chalk, than that the hens created the 

 lime of which their egg-shells consist. Chalk falling 

 into the sea would speedily become animated with 

 multitudinous forms of life there existing : and the 

 rapid development of those forms of life is such as 

 almost passes imagination. If under the microscope 

 many portions of chalk exhibit minute marine shells, 

 this would show that the chalk, where it abounds 

 in these, had once lain as a deposit at the bottom of 

 the sea, and had been so used while there : but if 

 other portions of chalk should be found, not con- 

 sisting of nor containing minute shells, this would 

 again demonstrate the fallacy of such a theory of its 

 origin. No doubt shell-fish derive the material of 

 their shells from the sea-water containing a diffused 

 portion of lime ; but their shells are not properly 

 chalk, nor, I apprehend, would any accumulation of 

 their shells form a chalk cliff. On the other hand, 

 if a stream of carbonic acid gas be passed through 

 lime water, it will form a deposit of chalk quite 

 without the intervention of marine insects ; and if 



