152 THALLOPHTTA. [CH. 



recorded the occurrence of these lowly organised plants. He 

 writes, — " I found a whole world of diatoms and other micro- 

 scopical organisms, both vegetable and animal, living in the 

 fresh-water pools on the Polar drift-ice, and constantly travelling 

 from Siberia to the east coast of Greenland'." In warmer 

 latitudes diatoms abound in the surface waters, but there 

 they are associated with numerous other forms of the Plankton 

 vegetation. The waters of the Amazon carry with them into 

 the sea large numbers of fresh-water forms, which are floated 

 out to sea and finally added to the rock-building material which 

 is constantly accumulating on the ocean floor^. No definite 

 results have so far been obtained as to the geographical and 

 bathymetrical distribution of marine diatoms. 



The enormous number of recent species precludes any 

 attempt to give a description of the better-known forms. It 

 is more important for us to realize how common and widely 

 distributed are the living genera. The hard and almost 

 indestructible valves have been frequently found in a fossil 

 condition, often forming thick and extensive masses of siliceous 

 rock. From diatom-beds now forming in lakes and on the 

 ocean-bed we pass to deposits such as those in Skye and 

 elsewhere, which mark the site of recently dried-up sheets of 

 water, and so to older rocks of Tertiary age formed under 

 similar conditions. Among the many examples of diatomaceous 

 deposits of Tertiary and Cretaceous age mention should be 

 made of those of Berlin, Konigsberg, Bilin in Bohemia, and 

 Richmond in Virginia. The diatoms in the beds of Berlin are 

 regarded as fresh-water, and those of Richmond as marine. It 

 has been pointed out by Pfitzer that it is a comparatively easy 

 matter to distinguish between fresh-water and marine forms 

 of diatoms. The diatomaceous rocks of Bilin are known as 

 polishing slates; they attain a thickness of -50 feet. In these, 

 as in many other cases, the deposit has become cemented 

 together as a hard flinty or glassy rock, in which the cementing 

 material was formed by the solution of some of the diatom 

 tests'. In many cases in which calcareous and siliceous rocks 



1 Nansen, Daily Chronicle, Nov. 2, 1896. ^ gchiitt (93) p. 10. 



3 Ehrenberg (36) p. 77. 



