NATURAL HISTORY OF THE BANKS OF THE TAY. 
37 
become the home of the bleaching and dyeing indnstries, but it may 
be less generally understood that the quality of the water is specially 
suited to these purposes. The exact connection between the water 
and the industries can probably be explained but by few. Without 
going deeply into chemical details, it may be stated that two out¬ 
standing qualities of Tay water are its softness and its purity. Soft¬ 
ness is accounted for by freedom from dissolved salts, especially those 
of lime, and purity is caused by freedom from sediment. Through 
almost the entirety of its highland course the bed of the river is 
composed of crystalline schists, which contain only slightly soluble 
mineral matter, and limestone is absent except for a few narrow 
bands, such as those on Loch Tay and on the Tummel, which are 
quickly passed over, and yield but a trifling quantity of material. In 
fact, the quantity of dissolved salts and lime contained in the water 
is only 3 to 4 grains to the gallon—an exceptionally small amount. 
Even the Thames, the water of which cannot be classed as the hardest, 
contains as much as 2c grains to the gallon. Again, the rocky 
nature of its bed sufficiently accounts for the small quantity of fine 
mud brought down by the Tay, for the extent to which it passes over 
beds of alluvial clay, at least previous to reaching Perth, is compara¬ 
tively insignificant, and gives little opportunity for gathering the finer 
sediment which might remain long in suspension. The average 
quantity of sediment brought down has been computed to be at 
Newburgh, 6|- tons per minute in a total discharge of 7,500 tons per 
minute, equal to i in 1,100 by weight. 
SALINITY. 
A new element enters into the composition of all rivers as they 
approach the sea, viz., the intermingling of the salt water of the 
ocean itself. The phenomenon of salinity is so gradual in its appear¬ 
ance, and varies so greatly according to circumstances—such as state 
of tide, volume of current, &c.,—that it is impossible to fix a definite 
point where its presence can be first detected. In the case of the 
Tay it has been asserted that salt has been noticed so far up as Kin- 
fauns, but the evidence for this is not reliable, and it seems more 
probable that the sea water does not penetrate farther up than 
Newburgh, if so far. Salt was detected in June, 1885, at a highest 
point only five miles east of Newburgh, but in this department 
there is still room for careful observation. Travelling seaward, 
there is found to be off Dundee a mixture of about equal parts of 
sea and river water. But it is a characteristic of the Tay that the 
fresh water has a tendency to remain on the surface, and this is found 
to be the case in St. Andrews Bay, and even beyond the Bell Rock, 
where the salinity increases with the depth. 
