DEPOSITS OF THE SCOTTISH FRESH-WATER LOCHS 267 
would be difficult to account for the loosely combined iron in it except 
by precipitation. From the experiments of Spring^ we know that 
dissolved humus and ferric iron coagulate each other undei- the action 
of sunlight, whilst on the other hand there is certainly much iron in 
the ferrous state dissolved in peaty water. It appears probable, then, 
that alkali-humus is a chemical precipitate of insoluble iron humate, 
which has been sent down from the upper waters by the oxidation of 
a soluble iron compound, the latter being either ferrous bicarbonate 
or a combination of ferrous iron with perhaps a quite distinct form 
of humic acid. The precipitate is not of a permanent nature, but is 
slowly oxidised away, whilst the iron contained in it goes back into 
solution in the ferrous state ; in support of this the absence of humus 
in the clay underlying Brown Mud, and the exudation of iron from 
wet Brown Mud samples (see above, p. ^64), may be mentioned. 
Iron thus seems to act as an oxygen -carrier in the breaking-down of 
vegetable debris. 
The elimination of organic matter from a loch, however slow the 
process may be, and whatever its mechanism, must depend in the last 
resort on the supply of atmospheric oxygen to the loch waters. There 
rarely seems to be a sufficient excess of oxygen in the deepest waters 
of lochs to keep the bottom clean by purely chemical oxidation ; and 
the disposal of vegetable debris by direct fermentation into methane 
and carbon dioxide'^ is apparently inhibited, or much hampered, by 
peaty water. Hence in depressions of the bottom, where vegetable 
debris tends to collect by simple gravity, there is sure to be a shortage 
of oxygen, and Brown Mud is the staple deposit. Small lochs, again, 
which receive more vegetable refuse per unit area than large ones, 
may be, and often are, wholly carpeted with Brown Mud. Wherever 
Brown Mud is absent on the floor of a loch it may be inferred that 
there is either a brisk movement of the bottom water, a copious supply 
of dissolved oxygen, or a gradient too steep to afford lodgment to 
vegetable debris. 
A special variety of this class of deposit may be termed Sulphur- 
etted Mud ; it consists of Brown Mud containing ferrous sulphide, 
and is characterised by a colour approaching black and a smell of 
sulphuretted hydrogen. Free sulphur, due to the partial oxidation 
of sulphides, is always present. Sulphuretted muds are occasionally 
met with in the large Highland lochs, but only in the deepest hollows, 
or at an inflow of drainage from an inhabited spot. In the small 
lochs of the Hebrides, Orkney, Shetland, and the lowlands, where the 
water is as a rule comparatively stagnant and not peaty, they occur 
very frequently, and in many cases constitute the sole deposit. Their 
1 Bull. Acad. Belg., t. xxxiv. p. 578, 1897. 
- Hoppe-Seyler, Zeitschr. Physiol. Chem., Bd. x. p. 401, 1886. 
