THE FRESH-WATER LOCHS OF SCOTLAND. 
425 
As there is no clayey matter in this sediment, and as the micro- 
scopical investigation did not reveal the presence of many aluminous 
minerals, the high percentage of alumina, i.t. 15*12 per cent., is to be 
explained by the fact that the ferrugineous grains are not made up of 
pure iron oxide, but of a mixture of this oxide with clay. 
Although manganese was not estimated in the above quantitative 
analysis, it was found to be rather abundant in another sample. 
(3) Peaty Mud. 
This type of mud occupies a large area of the floor of the loch in the 
south-west end of the Invermoriston deep basin. 
In order to show the differences in composition due to increase of 
depth, two descriptions will be given here. 
First Sample — Depth 300 feet. 
Minerals (35 per cent.), angular, mean diameter 0*2 mm. : orthoclase 
and acid plagioclase, greenish chlorite in large flakes, quartz, horn- 
blende, and ferrugineous matter. 
Fine washings (65 per cent.), composed of vegetable matter (37*10 
per cent.), and mineral particles (27*90 per cent.) belonging to the 
above-mentioned species.' 
Chemical Composilioji. 
Total silica ... ... ... ... ... 47 *88 
Ferric oxide ... ... ... ... ... ... ... 5*58 
Alumina 7*02 
Lime ... ... ... ... ... ... 1*08 
Magnesia ... ... ... ... ... ... ... 0 59 
Loss on ignition ... ... .. ... ... ... 37*10 
99*25 
Second Sample — Depth 445 feet. 
Minerals (10 per cent.), angular, mean diameter 0*12 mm. : ortho- 
clase and acid plagioclase, quartz, chlorite, hornblende, and ferrugineous 
matter. 
Fine washings (90 per cent.), composed of vegetable matter (25 per 
cent.) and fine mineral particles (65 per cent.) of the same species as 
those mentioned under the heading : minerals, but chlorite and decom- 
posed felspar are relatively more abundant. 
Chemical Composition. 
Total silica ... ... ... ... ... ... ... 46*03 
Ferric oxide ... ... ... ... ... ... ... 10*41 
Alumina 7*61 
Lime ... ... ... ... ... ... ... ... 9*64 
Magnesia 1 *60 
Loss on ignition ... ... ... ... ... ... 24*65 
99*94 
