264 THE FRESH-WATER LOCHS OF SCOTLAND 
from affluent rivers ; they form the substratum of all lacustrine 
deposits, and it depends largely on hydrodynamical conditions whether 
they shall remain exposed or become covered with another kind of 
deposit. Wherever the water is in gentle motion, sufficient to wash 
away organic debris but not vigorous enough to produce sands, there 
will be a surface deposit of Clay. The sedimentation of Clay is 
continually proceeding all over a loch (except at parts of the shore- 
line), but most rapidly at the embouchures of affluents. On comparing 
lacustrine with oceanic Clays one is struck bv the comparative paucity 
of argillaceous matter in the former ; this is due partly to the greater 
age of the ocean, in which the chemical degradation of silicates has 
been able to progress further than in lakes, and partly to the tendency 
of true Clay to remain suspended in fresh, as distinct from salt, water, 
and to be carried out to sea. Again, the Clays of the Scottish lochs 
are conspicuously less ferruginous than those of the deep sea, which 
is doubtless due to the marked solvent power on iron compounds of 
peaty waters. Although much of the iron thus extracted returns to 
the loch deposits in organic combination (Brown Mud), the greater 
part is probably lost irrevocably to the mainland and eventuates at 
the bottom of the sea.^ 
3. Brown Muds 
Brown Mud is the Scottish loch deposit par excellence. Its 
characteristic constituent is an impalpable brown humus-like product 
of the decay of vegetable matter. This substance usually shows no 
coarse remnants of tissue and is quite amorphous, though often 
coagulated into tiny balls. It is found occurring mingled in all 
proportions with the other kinds of loch deposits. Brown Muds, 
even the most typical, are never purely organic, but are invariably 
mixtures of vegetable and mineral detritus. The organic component 
contains iron (and a little manganese) in combination, and when wet 
Brown Muds are preserved in bottles iron is often found to be leached 
out, and to form a scarlet limonitic scum at the upper surface in 
contact with air. 
As to the chemical nature of the organic component of Brown 
Mud little can be said, in view of our scanty knowledge of the 
chemistry of humus. It is separable into two distinct portions. 
The one, which may be referred to as alkali-humus, dissolves 
readily in dilute alkalies or ammonia, giving a deep brown solution. 
From this solution acids bring down a dark brown flocculent substance 
which is very sparingly soluble in water or alcohol, dissolves partially 
in glacial acetic acid, and contains iron. A specimen of alkali-humus 
1 Of. Murray and Irvine, Proc. Roij. Soc. Edin., vol. xviii, p. 240 (footnote), 189L 
