DEPOSITS OF THE SCOTTISH FRESH-WATER LOCHS 269 
constituent, because one would on theoretical grounds ^ expect the 
silica of fresh-water Diatoms to be slightly more hydrated than that 
of marine Diatoms ; the latter is supposed to attach to itself about 
8 per cent, of water. ^ 
The Diatom species dominant in these five deposits, for the 
determination of which I am indebted to the courtesy of Professor 
G. S. West, of the University of Birmingham, are the following : — 
Lochindorb . . . Navicula major. 
Sur'irella rohusta^ var. splendida. 
Loch Allt an Fhearna . Mclosira distans. 
Surirella 7'ohusta^ var. splendida. 
Loch Assynt . . . Epithemia Hyndmanni. 
Loch an Duna . . Cydotella compta. 
Surirella rohusta^ var. splendida. 
Loch Frisa . . . Cyclotella compta^ var. radiosa. 
5. OcHREous Muds 
Ochreous deposits are distinguished by a high content of limonitic 
iron, which gives them a pronounced red colour. Besides limonite 
they contain the inevitable admixture of Clay, and always more or 
less organic matter ; as the proportion of the latter decreases, the 
colour becomes more brilliant. Good examples of Ochreous Muds 
are found in Loch Ness, 600 feet ; Loch Laoghal, 51 feet ; and Loch 
Assynt, 77 feet. An analysis of the first-named (" P'erruginous Mud ") 
has already been published;^ it shows per cent, of total ferric 
oxide. The Loch Assynt sample is the purest, i.e. most ferruginous, 
specimen hitherto met with, and has a fine Venetian red tint ; the 
organic matter (see above, p. 265) is trifling in amount. A mineral 
analysis of this deposit gave the following results : — 
Total ignition loss 
12-68 
Si02 . . . 
13-60 
Al^O, 
13-62 
Fe^^Og 
55-49 
MnO, . 
1-89 
CaO " . . . 
0-86 
MgO 
0-92 
K,0 
0-95 
Na/) 
0-57 
100-58 
^ Spring and Lucion, Zeitschr. anorg. Ohem., Bd. ii. p. 195, 1892. 
2 Ghalle7iger Report on Deep-sea Deposits, -p. 212, 1891. There is, however, room 
for revision here ; the analysis on p. 212 points to 6^ per cent, rather than 8 per cent. 
^ Geogr. Journ., vol. xxxi. p. 58, 1908. 
