40 THE MICROSCOPE. Marcli 



Why There are no Baccillariaceae in the Chalk. 



By ARTHUR M. EDWARDS, M. D. 



NEWARK. N. J. 



It is accounted for now why the shells of BacillariacesB 

 are not present in the chalk. Of course many times 

 chalk has been searched for Bacillariacese, Diatoms, but 

 always without success. Radiolaria have been looked for 

 until lately unsuccessfully. Sponges were seen rarely 

 and flint, which was suspected to have been sponges like- 

 wise, were common. But now a paper by Messrs. Hill 

 and Jukes-Browne, on the occurrence of Radiolaria in 

 chalk solves the problem. The chalk was commonly 

 acted upon by acids, hydrochloric or nitric acid. It was 

 supposed that all calcareous matter would be dissolved 

 and siliceous left. Forgetting that the silica was present 

 in the shells of organic beings, and also that the silica 

 was present. as colloid silica, which is readily soluble 

 in ordinary water, as crystalline silica is not. Messrs- 

 Hill and Jukes-Browne were surprised to find the variety 

 of the shells of Radiolaria in the chalk of the upper Cre- 

 taceous age, seeing that they are not uncommon in the 

 lower Cretaceous rocks, and alsothey have been found in 

 beds of the Jurassic age. As they say it will be seen that 

 one cause of their rarity is uildoubtedly the facility with 

 which the siliceous skeletons of Polycistina disappear 

 when they are embedded in rocks which contain a large 

 proportion of carbonate of lime. They examined several 

 hundred slides cut from sections of all the various divisions 

 of the chalk, from many different localities, but nothing 

 ever presented itself in any of these which was unmis- 

 takably a test of shell of a Radiolaria. This was several 

 years ago. Then they examined some of the Barbados 

 rock, the Infusorial roCk as it is known, which has such 

 beautiful Diatoms in it with Polycistina, Radiolaria also. 

 This rock was siliceo-calcareous, that is to say it had 



