Bituminoid Cement of the Caithness Flagstones. 321 
various substances brought down by the rivers in solution, 
being only partially absorbed by the earth, and not at all 
carried off, as the water is, by evaporation, tend to accumulate 
in solution, and do so, until their several points of precipitation 
are reached, when, of course, each in its turn goes down 
in the solid form. Rock-salt is one of these. [In regard to 
this, I have once before stated before the Royal Physical 
Society my belief that the occurrence of this mineral is often 
due to the following sequence of events :—Firstly, it has 
been driven up from the ocean in the form of spray during 
storms, and in a state of minute crystals has been subse- 
quently diffused by the agency of aerial currents through the 
atmosphere. Secondly, that these particles of chloride of 
sodium diffused through the air have formed nuclei around 
which aqueous vapour has been condensed. Thirdly, in the 
form of rain it has again descended to the earth, whence, 
under normal conditions of drainage, it has found its way 
back again to the sea. This part of its history has been 
made abundantly clear by the labours of Mr John Aitken of 
Darroch. In the case of areas of inland drainage, however, 
as I have pointed out, the cycle is not completed at once, 
for the chloride of sodium does not immediately return to 
the sea, but remains inland, where I believe that, in time, 
it forms one of the sources of beds of rock-salt.] Bicar- 
bonate of lime is a second constituent carried in solution 
in river-waters, and sulphate of lime is a third. Both 
of these play an important part in connection with the 
Caithness Flags. Along with these two substances, most 
rivers contain in solution more or less ferrous carbonate 
and sulphate. The first of these is due to the action of 
the humus acids and their ultimate term carbonic acid, 
which have disengaged the iron, in the first instance, from 
its primary source, the iron-bearing minerals of eruptive 
rocks. Disregarding for the moment these substances in 
solution, we may pass on to notice the character of the sub- 
stances of organic origin which are carried down in suspension. 
In areas of inland drainage there is usually a rainfall below 
the average in amount. This may range from a maximum of 
say 20 inches to a minimum of 10 inches, or even less than 
