DYNAMICAL GEOLOGY. 243 



When the river-water was low, only the most buoyant plant 

 detritus could be floated across the bar ; when the water level 

 was high, sand and pebbles were also carried into the basin of 

 deposit (ante, p 220). 



Lake-deposits of siliceous earth ("kieselguhr") were dis- 

 covered by Ehrenberg in 1837 to be composed of 

 the silicified valves or fragments of valves belonging to 

 unicellular plants of microscopic size, the Diatomacese. 

 These plants exist both in fresh and salt water, and their 

 remains have gathered on the floor both of inland lakes and 

 the ocean. Ehrenberg first demonstrated the presence of 

 diatom remains in the ground of Berlin, in the peat-mosses 

 of the Liineburg heath, afterwards in samples of pelagic 

 deposits, and in the "kieselguhr" and "tripoli powder" of 

 Bilin in Bohemia, Richmond in Virginia, and other places. 

 The explorations of the Challenger Expedition proved that 

 extensive areas of the ocean-floor were covered by the skeletons 

 and fragmentary debris of diatoms. In 1889, Weed found that 

 the separation of silica from the hot springs and geysers of 

 the Yellowstone Park was largely accomplished by diatoms. 



More important is the assistance rendered by certain plants 

 to the elaboration of limestone. It has long been known that 

 the formation of calcareous tufa is promoted by the growth of 

 moss, rushes, and certain algae. On the other hand, it was 

 discovered comparatively late in the history of research that 

 marine limestones sometimes attaining great thicknesses owe 

 their origin to algal organisms. Philippi was the first to 

 recognise, in 1837, that the pelagic Nullipores previously 

 regarded as polyps or Bryozoa belonged to the group of Cal- 

 careous Algae. The name of Nullipores was changed to Litho- 

 thamnia and Melobesia, and Unger in 1858 demonstrated the 

 important part such organisms had played in the construction 

 of the Leitha limestone in the Vienna basin during the 

 Miocene period. Two important works on the subject were 

 contributed and laid before the Bavarian Academy of Sciences 

 by Giimbel in 1871 and 1872. These works not only added 

 to the microscopic knowledge of the skeletal structures of the 

 Lithothamnian group, but also proved that other skeletal 

 remains widely distributed in the Alpine limestones, and 

 which had been referred by Schafhautl to the Bryozoa under 

 the name of Diplopores, agreed with the structure of the 

 Dactylopores. 



