Geological Society of London. 111 
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 recognizable 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 
Prof. Ehrenberg of Berlin, respecting the origin of tripoli. 1 need 
scarcely remind you, that tripoli is a rock of homogeneous appear- 
ance, 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 favorite 
theory of some geologists that it was a siliceous shale hardened by 
heat. The celebrated tripoli of Bilin in Bohemia consists of sili- 
ceous 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 tripoli 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 skeletons of micro- 
scopic animalcules. It is probably known to you, that this distin- 
guished physiologist has devoted many years to the anatomical in- 
vestigation 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 siliceous shells, or cases of pure 
silex. The forms of these durable shells are very marked and vari- 
ous, 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 fresh-water, and are 
all extinct. The tripoli of Cassel appears to be more modern, and 
the infusoria in that place, which are also fresh-water, 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 belong 
to well-known recent marine species. 
, 
