FOSSIL INFUSORIA. 39 



stance of the chalk, any decaying sponges, Al- 

 cijonia, sea-urchins, or other animals placed 

 there, would be sure to collect the particles of 

 the silex around them, and thus be converted 

 into flints/ 3 " We refer to beds of 



greater or less thickness, composed exclusively 

 of the solid' remains of animalcules creatures 

 individually so small, that only a microscope 

 could enable human eyes to see them. Such a 

 rock (called Tripoli) is found at Biliu, in Bohe- 

 mia,* and at Planitz, in Saxony. It has been 

 used as a powder in some of the arts, for ages, 

 without any suspicion of its being thus composed. 

 But within the last few years M. Ehrenberg, a 



* The series of strata forming this polishing slate is about fourteen 

 feet thick. 



