CHAP, xvi SOIL FORMED UNDER WATER 65 



In fresh-water, owing to the advent of material brought by rivers, 

 there arise deposits whose nature is determined by the geological character 

 of the soils traversed by the river. ' Pollen-mud ' arises by the accumu- 

 lation of pollen derived from anemophilous trees, such as conifers, beech, 

 and the like. 



Other deposits are produced by chemical processes, more or less due 

 to the activity of organisms. 



Calcic carbonate : In many spots in lakes a portion of the carbonic 

 acid in the water is decomposed mainly, it appears, through consumption 

 by green plants. The result is a precipitation of calcic carbonate, first, 

 on plants (Potamogeton, Characeae, and certain algae), partly in their 

 cell-walls ; and subsequently the precipitated chalk accumulates on the 

 lake-bed. Characeae may contain as much as 80 to 90 per cent, of 

 chalk. 



Compounds of iron are very often deposited, with or without the 

 co-operation of bacteria, Blue-green Algae, and other plants. 



Deposits mainly constituted of the remains of plants and animals 

 occur particularly in calm lakes, pools, and the like. They appear to be 

 most frequent in cold-temperate regions. Some (termed ' Gytja ' in 

 Scandinavia) are structureless, grey or brown masses, largely composed 

 of the more or less disintegrated excrement of animals, also of the remains 

 of small animals and plants. A large amount of fatty oil occurs in these 

 deposits, according to Potonie. A very important constituent of this 

 kind of soil comes from plankton. In lakes where diatoms abound these 

 are deposited in great quantities. In other cases it is mainly Blue-green 

 Algae, or the chitin of small freshwater crustaceans that are accumulated. 

 By chemical processes these common deposits undergo change as the 

 organic constituents are reduced. This ' Gytja ' is a kind of humus- 

 production under water. 



Another kind of deposit that is amorphous, jelly-like and brown 

 when moist, but more black when dry, arises especially in shallow waters 

 where the water is brown with humus and the vegetation is rich ; 

 with this type of deposit nymphaeaceous vegetation is particularly asso- 

 ciated. 1 



CHAPTER XVII. ARE THE CHEMICAL OR THE PHYSICAL 

 CHARACTERS OF SOIL THE MORE IMPORTANT? 



WE have already learnt that there are numerous differences in 

 the chemical and physical characters of soils, that is to say, in the 

 amount and kind of the components, and in the water-capacity, tenacity, 

 and so forth. Varied combinations of these bring into existence 

 extremely diversified types of soil. 



1 Our knowledge of these various deposits, connected as they are with one 

 another by very gradual intermediate types, is very defective. They were first 

 studied by H. von Post, 1862 ; and more recently investigated by Ramann, 1895 ; 

 Weber, 1903 ; Potonie, 1905 ; Fruh and Schroter, 1904 ; Wesenberg-Lund, 1901 ; 

 Ellis, 1907. 



WARMING p 



