ALG^ AS ROCK-BUILDING ORGANISMS. 



FROM the point of view of a botanist the phycological 

 records of the rocks are exceedingly disappointing. 

 The Lower Palaeozoic strata have afforded such genera as. 

 Nematophycus and Pachytheca, which may perhaps be re- 

 garded as extinct forms of algae, but cannot as yet be 

 assigned to any definite position in that class of plants. 

 The tissues of these two genera have been described by 

 various observers, and botanists have exercised consider- 

 able ingenuity in their speculations as to the exact position 

 of both fossils in the vegetable kingdom. 



Apart from Nematophycus and Pachytheca our know- 

 ledge, such as it is, of fossil algae is limited to structureless 

 specimens of various forms and sizes, which have been re- 

 ferred to algal genera on the evidence of any superficial 

 resemblance to the bodies of livinpf forms. 



The calcareous algae afford a. notable exception to the 

 general rule as regards the unsatisfactory state of fossil 

 phycology. Their hard parts have left well-preserved re- 

 mains in rocks of different ages, and have supplied trust- 

 worthy data for the study of extinct forms. In addition to 

 the calcareous algae, and a very few isolated examples of 

 non-calcareous genera, there are but few records which can 

 be utilised in phylogenetic studies. When we approach the 

 subject of fossil algae from a geological standpoint, we are 

 confronted with a mass of facts, collected for the most part 

 in recent years, which reveal the great importance of algal 

 remains as rock-building agents. It would be superfluous 

 to draw attention to such diatomaceous deposits as occur 

 in the Cretaceous and Tertiary formations, or to describe 

 the various conditions under which diatoms are now building 

 up siliceous rocks. If we accept Heer's genus Bactry 'Ilium 

 as an unusual type of diatom we may extend the geological 

 history of these rock-builders to Keuper and Muschelkalk 

 times, but from older strata trustworthy records have still to 

 be sought for. 



