DEPOSITS OF THE SCOTTISH FRESH-WATEB LOCHS 273 
of the floor of the ocean is covered with dead calcareous shells 
(Globigerina Ooze) fallen from above, and large areas also are 
composed of siliceous skeletons (Diatom Ooze and Radiolarian Ooze). 
These oozes consist of inorganic material which has been precipitated 
by biological agencies out of solution in the sea. Such deposits play 
but an insignificant part in lakes. It is clear that, even if the requisite 
forms of life were present, there could not be much precipitation of 
calcium carbonate in lakes, seeing that there are only 30 to 40 parts 
per million of calcium in normal soft lake- water, whereas in sea-water 
there are about 400 parts per million. Apart from the sparse frag- 
ments of large shells occasionally found near shore, calcareous matter 
seems never to exist in the deposits of Scottish mainland lochs. 
Such calcareous deposits as have been found in island lochs are 
products of vegetable life, and are thus comparable to the coccoliths 
and rhabdoliths of deep-sea deposits. Calcium carbonate secreted by 
plants, especially algas, is a not unimportant constituent of Danish 
lake deposits.^ On the whole, however, it may be said that, whilst 
submarine lime is wholly of biological, that of lake-deposits is mainlv 
of detrital, origin. 
Diatomaceous deposits are occasionallv met with in the lochs, as 
recorded above. As compared with oceanic Diatom Oozes, they con- 
tain more clayey silt, and they are free from the lime which invariably 
accompanies their oceanic analogues. There is reason to believe that 
peaty water such as that of the lochs is, if anything, richer in silica 
than that of the ocean. If, in spite of this, diatomaceous lake-deposits 
are somewhat uncommon, the reason may be either that sea-water is 
relatively a kindlier medium for this form of life, or that dead Diatom 
frustules are redissolved more rapidly in loch water. 
Peaty water carries in solution a certain amount of iron existing 
as soluble humate, a solute which is absent or insignificant in ocean- 
water. Brown Mud, as we have seen above, appears to be partially 
derived from this source, and would in so far be classifiable as a 
precipitated deposit. Anything like Brown Mud is unknown at the 
bottom of the open sea, since precipitated humus is rapidly cleared 
away by bottom-living animals. A comparison of Loch Ness with 
a similarly shaped and environed salt-water loch, such as Loch Fyne, 
is instructive in this respect. Both lakes exist under similar condi- 
tions, but the former holds fresh water and the latter sea-water ; the 
result is that Brown Mud accumulates at the bottom of Loch Ness, 
whilst in Loch Fyne the bottom is kept comparatively clean and free 
from vegetable organic matter. 
(3) Decomposition of minerals is much the same process in oceanic 
as in fresh water. Alkalies, calcium, and magnesium are eliminated, 
^ Wesenberg-Lund, Studier over Sdkalk, etc., p. 154, Copenhagen, 1901. 
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