262 THE FRESH-WATER LOCHS OF SCOTLAND 
Three other types of deposit occur sporadically and are by way of 
being rarities, viz. : — 
(4) Diatom Ooze. 
(5) Ochreous Mud. 
(6) Calcareous Deposits. 
L Sands, etc. 
Wherever the bottom of a loch lies under briskly moving water, 
as within the sphere of wave-action or at the inflow of a rapid river, 
the deposits are graded by elutriation, and the finer material is carried 
away. The residue will consist of coarse and heavy mineral grains 
comparable to sea-sand. Sandy loch deposits, then, are only found in 
shallow depths, and usually near the shore-line. They consist chiefly 
of quartz, felspar, and mica, and are free, or nearly so, from clayey 
matter ; the more vigorous the elutriating agency, the more does 
quartz tend to predominate. Sandy deposits are often discoloured by 
organic matter, which is apparently not washed away so easily as 
clay ; also by limonite, existing as a tenacious incrustation on quartz 
grains. An analysis of a sand from Loch Ness, 30 feet, has been 
published in an earlier paper of the Survey.^ In the majority of 
Scottish lochs, which are more or less steep-sided and U-shaped in 
section, the layer of sandy deposit may be supposed to extend from 
bank to bank, underlying deposits of finer material in the inner part 
of the loch, and being itself underlain by yet coarser grains and 
pebbles. This scheme of stratification was well illustrated by some of 
the Survey soundings, in the rare cases when it was possible to bring 
up a long plug in the sounding- tube. As regards the origin of 
sandy deposits, it is clear, since they are too coarse to be transported 
to any extent by water, that they are derived from the rocks 
immediately surrounding the loch. They are, as it were, autochthonous, 
and differ in this respect from the material of the finer mineral 
deposits (Clays), which may in part have arrived from great distances. 
2. Clays 
The term Clay is here applied to any mineral deposit which is 
sufficiently fine-grained and coherent to have a certain plasticity in 
the wet state. In the Scottish lochs Clays and Brown Muds shade 
off' into one another through an infinity of gradations ; we may 
regard as typical Clays those specimens (and they are plentiful 
enough) which contain practically no organic matter, and are 
farthest removed from the Brown Mud end of the series. Such 
1 Geogr. Journ., vol. xxxi. p. 60, 1908. 
