266 
THE FRESH-WATER LOCHS OF SCOTLAND 
approximately the proportion of organic matter present, and afford a 
measure of the simultaneous deposition of humus and mineral silt. It 
will be seen that even the richest Brown Mud (No. 3) still contains 
fine minerals to the extent of about two- thirds. The discrepancy 
between the ignition-losses in the first column (over the blowpipe) 
and those in the fourth (at dull red heat) is due to the water of 
constitution of the micaceous residue. From the high ratio of water 
to carbon in Nos. 1 and 5 the presence of notable admixtures of 
argillaceous matter may be inferred ; the same disproportion in No. 2 
is accounted for by diatomaceous silica, and in No. 6 by limonite. 
The ignition-residue, representing mainly mineral silt, of the 
Brown Mud No. 3 showed on analvsis : — 
SiO 
. 56-62 
A1203 
. 21-59 
- . 13-43 
MnO 
0-94 
CaO 
1-47 
MgO 
1-28 
K,0 
. 4-29 
Na20 
1-08 
100-70 
Since the crystalline minerals present are known to consist almost 
entirely of quartz and mica, the very respectable content of manganese, 
and much of the ferric oxide, would appear to have been brought 
into the deposit along with the organic matter. A portion of the 
iron is, as we have seen above, an integral constituent of the humus- 
like sediment ; the clear caustic-soda extract from 1 gram of No. 3 
yielded, on precipitation with acid, 0-29 gram of alkali-humus, the 
ash of which contained 0*023 gram of Fe^Og. 
Brown Mud is found at all depths ; in the same loch it usually 
occurs, as a surface-deposit, at lower depths than pale Clay. In well- 
marked depressions of a loch-bottom, and under comparatively stagnant 
water generally, it is the invariable sediment. 
As to the origin of the organic constituent of Brown Mud, there 
can be no doubt that it is derived ultimately from the decomposition 
of vegetable matter. Much plant-refuse, such as leaves, twigs, etc., 
is imported into lochs, especially in autumn and winter, and a 
partially decayed compost of such is not uncommonly observed as a 
layer over the Brown Mud proper. The immediate remains of this 
refuse account for the organic deposit insoluble in alkali. But the 
dark amorphous ferruginous alkali-humus has all the appearance of 
having been precipitated, rather than formed m situ ; moreover, it 
