PART I. CHAPTER III. 



Infusoria in Tripoli. 



men is now converted into a stony mass, a mixture of clay 

 and lime ; but it must once have been buoyant and floating in 

 the sea, when the TeredinaB lived upon it, perforating it in all 

 directions. Again, before the infant colony settled upon the drift 

 wood, the branch of a tree must have been floated down to the 

 sea by a river, uprooted, perhaps, by a flood, or torn offatid cast 

 into the waves by wind ; and thus our thoughts are carried back 

 to a prior period, when the tree grew for years on dry land, 

 enjoying a fit soil and climate. 



It has been already remarked that there are rocks in the inte- 

 rior of continents, at various depths in the earth, and at great 

 heights above the sea, almost entirely made up of the remains 

 of zoophytes and testacea. Such masses may be compared to 

 modern oyster-beds and coral reefs ; and, like them, the rate of 

 increase must have been extremely gradual. But there are a 

 variety of stony deposits in the earth's crust now proved to have 

 been derived from plants and animals of which the organic ori- 

 gin was not suspected until of late years, even by naturalists. 

 Great surprise was therefore created by the recent discovery of 

 Professor Ehrenberg of Berlin, that a certain kind of siliceous 

 stone, called tripoli, was entirely composed of millions of the 

 skeletons or cases of microscopic animalcules. The substance 

 alluded to has long been well known in the arts, being used in 

 the form of powdef for polishing stones and metals. It has been 

 procured, among other places, from Bilin, in Bohemia, where a 

 single stratum, extending over a wide area, is no less than 14 

 feet thick. This stone, when examined with a powerful micro- 

 scope, is found to consist of the siliceous cases of infusoria, united 

 together without any visible cement. It is difficult to convey an 



Fig. 14. Fig. 15. Fig. 16. 



These figures are magnified nearly 300 limes, except the lower figure of G. ferru- 

 ginea (Fig. 16, a,) which^is magnified 2000 times. 



idea of their extreme minuteness ; but Ehrenberg estimates that 

 in the Bilin tripoli there are 41,000 millions of individuals of the 

 Gaillonella distans (see Fig. 15.) in every cubic inch, which 

 weighs about 220 grains, or about 187 millions in a single grain. 

 At every stroke, therefore, that we make with this polishing 

 powder, several millions, perhaps tens of millions, of perfect fos- 

 pils are crushed to atoms. 



