150 
THE FRESH-WATER LOCHS OF SCOTLAND 
Turning now to lakes not furnished with outlets, we meet with a 
variety of waters which contain relatively high proportions of dissolved 
matter, and show great eccentricities in their chemical composition. 
Ordinary lakes are merely temporary storehouses of the soluble 
substances extracted from the surrounding land on their way to the 
sea. Terminal lakes, on the other hand, usurp the function of the 
sea itself as the final repository of these substances. In such lakes 
the loss of water by evaporation is ipso facto equal to, or greater 
than, the supply from affluents and rainfall. Consequently all the 
dissolved matter of inflowing rivers is gradually accumulated in the 
lake water, and, generally speaking, the older the lake, the more 
highly charged its waters will be. The total solids may thus rise to 
hundreds of parts per thousand. 
Profound alterations in the nature of the dissolved matter are 
involved by this process of concentration. In the first place, the 
predominant calcium carbonate of river waters is present only by 
virtue of free carbon dioxide in solution. Now this latter must 
remain approximately in equilibrium with the carbon dioxide of the 
atmosphere, and cannot, therefore, accumulate to any extent : on the 
contrary, it tends to be ousted more and more as the water increases 
in salinity. The result is that calcium carbonate must be eliminated, 
usually by precipitation as calcareous tufa, travertine, or oolite. In 
exceptional cases the oceanic mode of elimination, viz. by biological 
agencies, comes into play. The deposition of calcium carbonate 
begins at an early stage in the history of an enclosed lake, long before 
any other constituents are separated out. This transference of calcium 
from the dissolved to the undissolved state is one of the most salient 
phenomena of terminal lakes and of the ocean, and goes on con- 
tinuously so long as there is an influx of continental waters. 
The further changes which may take place as the concentration 
of lake water progresses follow from the solubilities of the various 
possible salts concerned. It will be remembered that any cation can 
pair off with any anion, so that as soon as the saturation-limit of a 
salt corresponding to any pair has been transgressed, that salt will 
begin to separate out. Leaving calcium carbonate on one side, we 
find that calcium sulphate would be the first salt to pass out of 
solution as gypsum or anhydrite. This precipitation only occurs at 
an advanced state of concentration, even when the lacustrine and 
affluent water is of the normal type. When the inflowing waters 
contain a slight excess of alkali, existing as sodium and potassium 
carbonates, calcium carbonate is thrown down by double decomposi- 
tion, and the separation of calcium sulphate is to a corresponding 
extent averted, whilst soluble sulphates accumulate in the water. 
When the total solids have passed beyond 20 per cent., sodium 
