1904 — 5 . ] Study of the Lakes of Scotland and Denmark. 437 
As far back as 1862 these layers were termed u gytje” in an 
admirable paper by the eminenl Swedish naturalist H. v. Post, and 
this term is very much used in North European and Danish 
literature. V. Post distinguishes different forms of “ gytje,” but 
we shall here only deal with the so-called “ Lake-gytje,” which is 
formed principally in clear, limpid water. In my paper (1901) I 
have pointed out that the main condition for the formation of this 
“gytje” appears to be, that no greater quantities of organic matter 
be precipitated than the bottom-fauna and the bacteria conjointly 
may be capable of digesting. If the supply of organic matter be 
superabundant, black fetid mud-formations (river-deltas, common 
sewers, etc.) result, while, on the other hand, where the organic 
matter, owing to the presence of humic acid, remains undecayed 
and is preserved, peat is formed. Owing to the digestive processes, 
the excrements are generally of a lighter colour than that of the 
lake-bottom itself. This might be accounted for by supposing that 
the animals of the upper layers feed mainly on the organic dark- 
coloured debris, allowing the inorganic matter, which in our lakes 
consists especially of lime and clay, to pass through their ali- 
mentary canals. By means of bore samples from shallow lakes I 
have shown that the colour of the lake-bottom grows lighter the 
deeper we go down ; it may be greyish-white 4 feet beneath a surface 
which is often quite black. I am of opinion that layers of almost 
pure lime or clay — so-called coprogenic lime and clay layers — 
may result from the digestive action of the bottom fauna and flora. 
With regard to the process of formation, these layers are not 
identical with those layers of clay which, during and immediately 
after the Ice Age, were formed on the primary sandy bottoms of 
our lakes, and were one of the first conditions for the development 
of a higher and more specialised organic life in the lakes. Nowa- 
days, in all our lakes, and probably in many of the lakes of the 
Central European plains, the precipitation of orgajnc matter — 
debris from the littoral zone as well as plankton — is very copious. 
In all our deeper lakes it is mainly the plankton which determines 
the composition of the lake-gytje; and as the plankton varies in 
the different lakes, it will be understood that the lake-gytjes 
consequently also differ from each other. 
In our lakes I have been able to distinguish three different 
