230 



PHYSIOGRAPHY. 



[chap. 



Sheppey. It must be borne in mind, that such fossils 

 form only an insignificant portion of the bulk of the rocks 

 in which they are embedded. There are, however, other 

 organisms which enter so largely into the composition of 

 certain deposits as to form by far the greater proportion of 

 their bulk. 



Thus, there is a well known substance, which has been 

 used in the arts for many years as a polishing materica, 

 under the name of Tripoli. It is a kind of rottenstone, 

 which occurs in large deposits in many parts of the world, 

 but is especially abundant at Bilin, in Bohemia, where it 



forms strata of considerable extent, 

 one bed measuring as much as four- 

 teen feet in thickness. In some 

 places, the tripoli is a soft friable 

 rock, while, in other localities, it is 

 so hard as to be known as " polish- 

 ing slate." Chemically, it is almost 

 pure silica, like the silica of rock- 

 crystal ; but examination under a 

 microscope shows, at once, that it 

 Fig. 63.— Fossil fruit (A';>«fl'//« is not simply mineral silica. Indeed, 



^///>^?V7*.y) from the London . t i r .^ • , • t • rc^ 



clay, Sheppey. whcn z. little of this tnpoli IS sufifi- 



ciently magnified, it is seen to be 

 made up, not of shapeless mineral particles, nor of minute 

 crystals of silica, but of such beautifully-formed objects as 

 those represented in Fig. 64. It was shown, many years 

 ago, by the late Prof. Ehrenberg, of Berlin, that these 

 delicate objects in the tripoli are identical with the siliceous 

 cases characteristic of a group of minute organisms called 

 Diatoms. The diatoms are found living both in salt and 

 in fresh water, but the kinds commonly preserved in tripoli 

 are characteristic of fresh water, and hence it is concluded 



