274 
THE FRESH-WATER LOCHS OF SCOTLAND 
water of hydration is taken up, and the result is clay. It has been 
mentioned that submarine clayey deposits are in a far more advanced 
stage of decomposition, i.e. contain far more clay proper, than 
lacustrine ones. This is due not to a more powerful corroding action 
on the part of sea-water, but rather to the greater geological age 
of submarine deposits. The continual transportation of fine suspended 
argillaceous matter from land into the ocean is also to be reckoned 
with. 
Except in quantities of 1 or 2 per cent, at the utmost, there 
is no organic matter in deep-sea deposits. The little (mainly animal) 
that reaches the bottom is rapidly oxidised away or consumed by 
the bottom-living fauna. The lochs are in a very different case. 
Here there is a plentiful influx of dead vegetable matter — more than 
the available supply of dissolved oxygen can cope with. This debris 
decays as far as the humus stage, instead of being broken down to 
carbonic acid ; the humus accumulates in combination with iron, 
and becomes in effect the characteristic lacustrine deposit. It is 
interesting to observe the vicissitudes of iron in the two media. In 
a loch we find the Clays much paler, that is, less ferruginous than 
deep-sea Clays, and a continual interchange of iron between the water 
and the bottom-deposits is going on ; whilst the Brown Muds lock 
up a good deal of iron (and manganese) and tend, if exposed to 
oxidising conditions, to become ever more ferruginous. The con- 
centration of iron as limonite or siderite in clay-ironstone and 
bog-ores is in fact peculiar to fresh water. In the sea, on the other 
hand, if we disregard the minor, and up to the present inexplicable, 
concentration of iron in glauconite, the career of this element is 
uneventful and ends with ferruginous Clay. 
It is scarcely necessary to point out that in lakes nothing similar 
to the vast areas of oceanic Red Clay, which substance is produced 
by the decomposition m situ of volcanic silicates, need be expected ; 
indeed, no lake is large enough to contain regions which, with 
respect to the deposits, might be termed " pelagic.'"' 
