146 THE FEESH- WATER LOCHS OF SCOTLAND 
soluble substances, both through the activity of its own waters and 
by drainage from the surrounding soil ; these substances are derived 
chiefly from the dissolution of rocks, with or without preliminary 
chemical decomposition, in which not only water but also carbonic 
acid and humic acid play a part. The general geochemical effect of 
flowing waters may be described as the action of a very dilute solution 
of carbonic acid on the earth's crust, resulting in a continual trans- 
ference of matter to the ocean. Thus it is impossible for any river 
to be quite free from dissolved solids ; but, on the other hand, the 
breaking down of rocks into soluble matter is an exceedingly slow 
process, and is very far indeed from keeping pace with the supply of 
pure meteoric water. Hence river waters, regarded as solutions, are 
necessarily of extreme dilution. The formation of anything like 
concentrated solutions only occurs in enclosed basins, e.g. the ocean 
or terminal lakes, as will be illustrated on a later page. Of the 
elements found in solution, the foremost are calcium, magnesium, 
potassium, and sodium, as ranking among the most abundant 
constituents of the lithosphere ; further, sulphur (as sulphates) and 
chlorine, as being, though less abundant, of highly soluble tendency ; 
whereas silicon, aluminium, and iron, the most abundant elements of 
all, are but insignificant items, owing to their insoluble tendency. 
Wherever there are sedimentary formations, that is, in most parts 
of the world, there is sure to be calcium carl^onate in some form ; 
this substance readily goes into solution, up to certain limits, in water 
containing carbonic acid, and, though it is very liable to be extruded 
by other solutes, or by removal of free carbonic acid, is to be regarded 
on the whole as the principal solute in rivers and fresh-water lakes. 
The precise form in which the various inorganic constituents exist in 
solution cannot here be dealt with at length. Broadly speaking, 
they are present not as definite salts, that is, combinations of an acid 
and a base, like sodium chloride, magnesium sulphate, etc., but as 
ions. Basic and acidic constituents, in fact, exist independently in 
solution, whilst salts as such are practically limited to the solid state. 
The result is that, if ever two ions, derived originally no matter from 
what salts, are in solution together to such an extent that the salt 
combined from them would be supersaturated, that salt is precipitated 
out of solution as a solid. 
The presence of calcium carbonate in a water which otherwise 
contains only sulphates and chlorides causes the water to show a 
weak alkaline reaction towards delicate indicators ; but it is im- 
portant to note that, when a water is spoken of as " alkaline,'* it 
owes its alkalinity not to calcium but to the alkali metals, sodium 
and potassium. These latter, when leached out of igneous rocks, 
are not accompanied by a strong acidic principle {e.g. CI or SO^) 
