INTRODUCTION 21 



ridge Clay, and some doubtful Dasydadacece from 

 other beds, there is no valid evidence of other forms 

 until the chalk is reached, with its Diatomacecc and 

 Lithothamnion in the Senonian (Cretaceous) beds, 

 Coccoliths, and Rhabdoliths. There come next the 

 extensive Tertiary deposits of diatoms, and the 

 beautiful verticillate Siphoncce of the same age 

 described by Munier Chalmas ; the coralline Litho- 

 thamnion and the fresh-water Characecc. The later 

 beds furnish little of interest except Quaternary 

 diatornaceous remains. There is room for dis- 

 appointment in the failure to find indubitable 

 records in earlier rocks of plants of such primitive 

 type as the Algae, and it is startling to find such 

 forms as the diatoms suddenly burst upon geological 

 history in a profusion of genera and species, many: 

 of which survive in their specific forms from 

 their first appearance to this day. From all that is 

 known of the Silurian rocks, for example, the 

 discovery of diatoms in them would appear to be 

 highly probable, but research has failed to discover 

 them. However intractable and therefore suited 

 to preservation their siliceous shells may be, the 

 existence of conditions under which this substance 

 would become fugitive is probable, and the gap, 

 though significant enough, is not more so than the 

 absence of Muscincce, for example, from the coal 

 measures. It but emphasizes the imperfection of 

 the geological record of plant history, and points to 

 caution in generalising from an insufficient array of 

 facts, more than it indicates argument in favour of 

 any particular sequence of primitive plant forms. 



