WHEEL ANIMALCULES. 173 



masses of earth consist wholly of the fossil remains 

 of animals of this class, the soft bodies of which 

 had been, during life, protected by a dense and 

 hard covering, or shield. These shields, which, 

 being formed of silica, were capable of resisting 

 the agency of an intense heat, have been preserved 

 in immense numbers, without change either of 

 form or of composition, during all the geological 

 revolutions which have taken place since the exist- 

 ence of their living inhabitants. They have been 

 found in different situations, in various parts of 

 Europe, in the Mauritius, and the United States 

 of America, forming, in their looser state, whole 

 beds of siUceous soil, well known by the name of 

 tripoli, or polishing earth, and which is extensively 

 used in the arts ; and constituting, in their more 

 compact form, whole masses of mountains.* Ves- 

 tiges of these animalcules may be also discovered 

 in the white siliceous material with which nodules 

 of flint, found in chalk-beds, are encrusted ; and 

 are also rendered apparent in the flint itself, when 

 properly prepared for microscopic examination. 

 These facts render it probable that the siliceous 

 stones met with in these situations, derive their 

 origin from the agglutination of the remains of vast 



* The size of each individual animalcule which composes the 

 Polirschiefer, or tripoli of Bilin, in Bohemia, is estimated by 

 Ehrenberg at about the 288th part of a line, or the 3456th of an 

 inch: and as this substance is slaty and without cavities, the ani- 

 malcules lie closely compressed. A cubic line would, therefore, 

 contain nearly 24,000,000; and a cubic foot above 41,278,000,000 

 animalcules; of which 187,000,000 are in weight only equal to a 

 grain. Those of the Raseneisen, or bog-iron ore, from Berlin, are 

 of considerably smaller dimensions, being only the 12, 000th of an 

 inch in diameter; so that 1000,000,000 would be contained in a 

 cubic line, and a billion in a cubic inch. 



