270 THE FRESH-WATER LOCHS OF SCOTLAND 
Almost all the iron was found to be capable of extraction by moder- 
ately concentrated hydrochloric acid (viz. 55*2 per cent. Fefi^). The 
predominant ingredient of the deposit was identified under the micro- 
scope as limonite in small amorphous grains, mean diameter 0*01 mm. 
There appears to be a genetic connection between Brown Muds 
and Ochreous Muds, inasmuch as examples of Brown Mud in pro- 
gressive stages of limonitisation are met with. This suggests that 
the ochreous matter may be a decomposition-product of ferruginous 
humus. Ochreous deposits are of highly localised occurrence, and 
must have originated either in a local supply of iron (as from 
chalybeate springs), or in a local concentration of iron pre-existing 
in the deposits or waters of a loch. For the former hypothesis there 
is no tangible evidence. It appears more probable that iron humates, 
dissolved in the loch-water or insoluble in the deposits, encountered 
a current of water strongly charged with atmospheric oxygen, reaction 
taking place with formation of limonite and liberation or destruction 
of humus-acids. As for precipitation of limonite from the water, 
this could only occur if there were extremely little organic matter in 
solution, since, so long as humic acid is in excess, not limonite, but 
insoluble iron humate, goes down.^ We are left to conclude, then, 
that Ochreous Muds are formed in situ by the oxidation of Brown 
Muds — whether directly, or through the agency of bacteria,^ must 
be left undecided. The humus of Brown Muds, as we have seen, 
holds ill combination a considerable amount of iron, which at one 
time must have been in solution in the loch-water, and at a still 
earlier stage must have been leached out of minerals by dissolved 
humic acids. It will be noted that the best examples of Ochreous 
Mud occur in deep water, where a mass of humus capable of leaving 
a tolerably thick layer of limonite may have had opportunity to 
accumulate. Whatever be the origin of lacustrine limonite, its 
formation and existence must certainly be bound up with an excess 
of dissolved oxygen in the adjacent waters. 
6. Calcareous Deposh's 
It will have been noted that no mention has been made in the 
foregoing pages of calcium carbonate, which plays so important a part 
in the deposits of the ocean and of lakes on the continent of Europe. 
Not a trace of this substance was found in any of the deposits from 
Scottish mainland lochs,^ and the reason clearly is that the country is 
^ Of. Spring, loc. cit. 
^ Cy. Van Bemmelen, Zeitschr. anorg. Ghem., Bd. xxii. p. 313, 1900, where 
references to the literature of iron-bacteria are given. 
2 Although there is a limestone formation of some magnitude at the eastern 
end of Loch Assynt, the twenty odd deposits from this loch, the floor of which 
