272 
THE FRESH-WATER LOCHS OF SCOTLAND 
(2) Precipitation, biological or purely chemical, from the over- 
lying waters. 
(3) Decomposition and synthesis of matter at the bottom. 
These agencies operate very differently in the lochs and in the sea, 
and the aqueous media also are very different ; hence it is not 
surprising that lacustrine and oceanic deposits should show more 
points of contrast than of similarity. 
(1) All the mineral matter at the bottom of a lake is derived by 
erosion from the surrounding country, either through direct wave- 
action on shore, or through the medium of affluent rivers. It is thus 
wholly terrigenous," whereas of the floor of the ocean only a limited 
strip around the continental coast-lines answers to this description. 
Inorganic lake deposits, then, are essentially similar to terrigenous 
oceanic deposits, consisting of clastic debris of continental minerals 
with more or less clay, and having a finer grain the greater their 
distance from brisklv moving water. Oceanic terrigenous deposits, 
however, undergo certain submarine modifications which are peculiar 
to sea-M ater as a medium : by the decomposition of (animal) organic 
waste within them they acquire an intimate admixture of fine 
carbonaceous matter, whilst the iron within them is partially reduced 
from the ferric to the ferrous state ; or, again, through the activity of 
bacteria which reduce the sulphates of sea- water, ferrous sulphide may 
be formed within them. These processes are carried out by the aid of 
bottom-living animals or bacteria, and produce the Blue Muds, a class 
of deposit which has no real analogue in the Scottish lochs, in the 
peaty waters of which there is no abyssal fauna to speak of and 
bacterial life is at a minimum. It is noteworthy that a blue-grey 
Clay resembling Blue Mud comes into existence at the bottom of the 
Lake of Geneva,^ where there is plenty of biological activity in the 
abyssal regions. On the other hand, lacustrine mineral deposits are 
subject to organic contamination of a kind unknown in the sea, in 
that they may become charged with the humus which results from the 
decay of vegetable matter. Humus in the Clay of the Scottish lochs, 
however, does not tend to be degraded further to the condition of the 
black carbonaceous constituent of Blue Mud ; this again may be 
accounted for by the absence of an abyssal fauna. As regards the 
mineral detritus in terrigenous deposits, it is qualitatively much the 
same in the lochs and the ocean. Detrital calcium carbonate, which 
does not exist in the lochs and does not reach the bottom in the ocean, 
occurs, however, in the lakes of Switzerland and Northern Europe as 
calcareous mud. 
(2) A most important factor in the formation of submarine deposits 
is the animal life with which the ocean swarms. More than a third 
1 Forel, Le Leman, t. i. p. 119, 1892. 
