VIl] FOSSIL DIATOMS. 153 



reveal no direct evidence of organic origin it is probable that 

 they were originally formed by the accumulation of plants of 

 which the structure has been completely obliterated by secondary 

 causes. The genus Gallionella plays an important part in the 

 composition of the Bilin beds. Occasionally impressions of 

 leaves and other organic remains are found associated with 

 the diatoms in the siliceous rocks. In the British Museum 

 (Botanical department) a large block of white powdery rock is 

 exhibited as an example of a diatomaceous deposit of Tertiary 

 age from. Australia. It is described as being largely made 

 up of the tests of fresh-water diatoms, such as Naviculay 

 Gomphonema, Cymhella, Synedra, and others. 



The abundance of Diatoms in Cretaceous rocks of the Paris 

 basin has recently been recorded by Cayeux^; it would seem 

 that these algae had already assumed an important role as 

 rock-builders in pre-Tertiary times. Cayeux points out that 

 the silica of these Cretaceous diatomaceous frustules has often 

 been replaced by carbonate of calcium. 



In addition to the occurrence of Diatoms in the various 

 diatomaceous deposits, their siliceous tests may occasionally be 

 recognised in argillaceous or other sediments. Shrubsole and 

 Kitton^ have described several species of Diatoms from the 

 London Clay of Lower Eocene age. In many localities in the 

 London basin the clay obtained from well-sinkings presented 

 the appearance of being dusted with sulphur-like particles of a 

 dark bronze or golden colour which glistened in the sunlight. 

 These yellow bodies have been found to be diatomaceous 

 frustules in which the silica has been replaced by iron pyrites. 

 The genus Coscinodiscus is one of the commonest forms recorded 

 from the London Clay^ 



Without further considering individual examples of dia- 

 tomaceous rocks we may briefly notice the general facts of 

 the geological history of the family. As Ehrenberg pointed 

 out several years ago, the Tertiary and Cretaceous species of 

 diatoms show a very marked resemblance to living forms. In 



1 Cayeux (92), (97). ^ Shrubsole and Kitton (81). 



' I am indebted to Mr Murton Holmes for specimens of these London Clay 

 Diatoms. 



