268 THE FRESH-WATER LOCHS OF SCOTLAND 
existence may be regarded as a certain indication that oxygen is not 
merely low in quantity, but altogether absent, in the bottom waters. 
The sulphur in them, which cannot well arise from the reduction of 
sulphates, as in sea-water,^ owes its origin to the decomposition of 
vegetable or animal proteids. Since animal debris is much richer in 
sulphur than vegetable. Sulphuretted Mud is the more likely to be 
formed, the greater the supply of animal matter from the loch-waters 
or from outside. It may be remarked that, however black and fetid 
such a mud may be, the actual content of sulphur is always exceed- 
ingly small,-^ as the following figures for two deposits reeking of 
sulphuretted hydrogen show : — 
Ferrous 
Sulphide. 
Free 
Sulphur. 
1. Kirk Loch, Lochmaben, 23 feet . 
2. Loch Harray, Orkney, 2 ,, 
0"17 per cent. 
0-22 
0*059 per cent. 
0-041 „ 
4. DiAT03i Oozes 
Whilst a stray Diatom here and there may be observed in almost 
any loch-deposit, especially in Brown Muds, patches of deposit occur 
in some lochs of which the bulk is composed of Diatom skeletons. 
Such deposits may be recognised at once by their lack of coherence. 
Of five Diatom Oozes noted, four are white, with a slight yellow 
discoloration due to clayey matter ; these are from Locb.indorb, 
Loch Allt an Fhearna, 15 feet. Loch Assvnt, 175 feet, and Loch an 
Duna (Lewis). The fifth, from Loch Frisa (Mull), 175 feet, is dark 
when wet, and dries to a mouse-coloured powder ; this deposit consists 
of Diatom frustules mixed with a good deal of humus, and quantita- 
tive particulars of its carbonaceous ingredient have been given above 
(p. 265). By extracting with very dilute caustic soda and determining 
the silica dissolved, information can be obtained as to the amount of 
diatomaceous silica present. Two of the deposits enumerated above 
gave the following figures for soluble silica : — 
Loch Frisa^ 175 feet^ . . 37'2 per cent. 
Loch Allt an Fhearna, 15 feet, . 68-1 „ 
The water combined with this silica cannot be satisfactorily 
determined unless exceptionally pure Diatom Oozes should happen to 
be available, and no attempt was made with the material at disposal. 
It would be interestmg, however, to know something about this 
^ Murray and Irvine, Trans. Roy. Soc. Edin., vol. xxxvii. p. 481, 1893. 
2 Cf. also Buchanan, Froc. Boy. Soc. Edi?i., vol. xviii. p. 17, 1891. 
