Mr. Lyell's Address, 409 



might infer that the same structure was preserved throughout the 

 mass. 



In my last address, I alluded to Mr. Lonsdale's detection of vast 

 numbers of microscopic corallines and minute shells in the substance 

 of the white chalk of various counties in England, where this rock 

 had not been suspected of consisting of recognisable organic bodies. 

 I cannot deny myself the pleasure of mentioning the still more sin- 

 gular and unexpected facts brought to light during the last year, by 

 Professor Ehrenberg of Berlin, respecting the origin of tripoli. I 

 need scarcely remind you, that tripoli is a rock of homogeneous 

 appearance, very fragile and usually fissile, almost entirely formed 

 of flint, and which was called polir-schiefer, or polishing slate, by 

 Werner, being used in the arts for polishing stones or metals. 

 There have been many speculations in regard to its origin, but it 

 was a favourite theory of some geologists that it was a siliceous 

 shale hardened by heat. The celebrated tripoli of Bilin in Bohe- 

 mia consists of siliceous grains united together without any visible 

 cement, and is so abundant that one stratum is no less than fourteen 

 feet thick. After a minute examination of this as well as of the tri- 

 poli from Planitz in Saxony, and another variety from Santa Fiora 

 in Tuscany, and one from the Isle of France, Ehrenberg found that 

 the stone is wholly made up of millions of siliceous cases and skele- 

 tons of microscopic animalcules. It is probably known to you, that 

 this distinguished physiologist has devoted many years to the ana- 

 tomical investigation of the infusoria, and has discovered that 

 their internal structure is often very complicated, that they have a 

 distinct muscular and nervous system, intestines, sexual organs 

 of reproduction, and that some of them are provided with sili- 

 ceous shells, or cases of pure silex. The forms of these dura- 

 ble shells are very marked and various, but constant in particular 

 genera and species. They are almost inconceivably minute, yet 

 they can be clearly discerned by the aid of a powerful microscope, 

 and the fossil species preserved in tripoli are seen to exhibit in the 

 family Bacillaria and some others the same divisions and transverse 

 lines which characterize the shells of living infusoria. 



In the Bohemian schist of Bilin, and in that of Planitz in Saxony, 

 both of them tertiary deposits, the species are freshwater, and are 

 all extinct. The tripoli of Cassel appears to be more modern, and 

 the infusoria in that place, which are also freshwater, are some of 

 them distinctly identical with living species, and others not. In the 

 tripoli brought from the Isle of France, the cases or shells all be- 

 long to well-known recent marine species. 



The flinty shells of which we are speaking although hard are 

 very fragile, breaking like glass, are therefore admirably adapted 

 when rubbed for wearing down into a fine powder fit for polish- 

 ing the surface of metals. It is difficult to convey an idea of 

 their extreme minuteness, but I may state that Ehrenberg esti- 

 mates that in the Bilin tripoli there are 41,000 millions of indivi- 

 duals of the Gaillonella distans in every cubic inch of stone. At 



Third Series. Vol. 10. No. 62. May 1837. 3 G 



