DEPOSITS OF THE SCOTTISH FKESH-WATER LOCHS 263 
lacustrine Clay is of a very light greyish or yellowish tint, and is 
much paler than any submarine inorganic deposit. It consists 
chiefly of finely divided quartz and mica, with minor proportions of 
felspathic, chloritic, and ferro-magnesian minerals.^ There is always 
present a certain amount of clay proper, i.e. amorphous hydrated 
alumino-ferric silicate, which imparts to the deposit its plastic 
character ; but the amount is often very small, and always much 
smaller than in oceanic Clays. 
Without resorting to an exhaustive analysis, an indication of the 
proportion of true clay in these deposits may be gained from their 
ignition losses. Organic matter being absent, ignition loss will 
represent the water of hydration of the clay present plus that of the 
mica present. Five samples of pale Clay, which gave little or no 
coloration with caustic soda solution, and were therefore regarded as 
free, or nearly so, from organic matter, were thus assayed ; they were 
weighed out after drying at 110° C. 
Deposit. 
Loss at Low 
Red Heat. 
Total Loss 
over Blast. 
Loch Assynt, 83 feet . 
Ness, 95 
,, Laggan, 59 ., 
,, Earn, 61 
Maree, 56 . , 
3-30 per cent. 
2-96 
3-93 per cent. 
1-95 
3- 48 
4- 24 
3-44 
Since ideal clay (Al2O3.2SiO2.2H2O) would give off over 14 per 
cent, of water on ignition, these figures speak for themselves. A 
rough attempt at discriminating between water of clay and water of 
mica was made on the last two samples, by igniting first at low red 
heat and then over the blast. If it were desired, the method might 
be made one of considerable accuracy by careful temperature 
adjustment, direct weighing of the disengaged water, and addition of 
sodium carbonate or lead peroxide in the final ignition. As it is, the 
Loch Earn sample shows about 23 per cent, of clay and 20 per cent, 
of mica, the Loch Maree sample about 21 per cent, and 11 per cent, 
respectively. 
Clays are met with in all the larger lochs. The absolute depths 
at which they occur are of course very variable ; but generally speak- 
ing they are characteristic of relatively shallow water, and they are 
never found, except as thick under-layers, at the bottom of deep 
basins. They constitute the natural silt or alluvium of lochs, com- 
posed of the geological detritus of the surrounding country. Clays 
may be introduced directly by erosion of the banks or indirectly 
^ For analyses of Clays and determinations of minerals see loc. cit. 
