ON DEPOSITIONS OF MUD IN THE TAY. 
7 
though pure water may percolate through them, that which contains muddy par¬ 
ticles will sustain a purifying filtration, leaving all its contaminating matter at 
or very near the surface of the Sand-bank. The waters of the river Tay when 
foul carry along with them nearly ~- Q th part of their weight of mud, and at 
all times probably 1.-th part. As the tide recedes, a free current of these will 
pass through the Sand-banks, which will then become covered with a fine film of 
silt, which, if left to the operation of precipitation by gravitation only, would 
most likely have been carried far out into the bosom of the ocean. The Sand¬ 
banks, being left dry for several hours every day, will leave the film thus 
accumulated to be acted on by the sun and air, so as to acquire a degree of 
consistency, which will make it rather tend to float away in a flocculent form on 
the next rise of the tide, than to mix with its waters diffused in the form it 
formerly assumed. In this case, if we suppose the process to be reversed, and 
the defiltrated tide that had previously descended through the emerging Sand¬ 
bank to rise through, the film will be floated upwards and onwards, and will 
most likely manifest the appearance formerly described. Besides this chief 
anomaly, several other difficulties are solved by this assumption. We see why 
no deposit of mud should take place above high-water-mark. Though there the 
river appears, and must necessarily be, most turbid, the mud is diffused through it 
in such impalpable particles, there is so great a state of agitation and onward 
motion, that they refuse to precipitate themselves. We can in this case show 
why no deposit takes place during the downward flow of the stream ; the mud is 
still too much diffused, till filtration gives it something like consistency of form. 
By this means we see at once how it should not be uniformly and imperceptibly 
precipitated, as we should at first sight expect, but falls in flakes to the bottom ; 
likewise, why wet weather and foods are unfavourable to deposition, the stream 
being sufficient either to sweep the banks of the filmy sheathing, or the rains 
which produce or accompany river freshes, being sure to keep the Sand-banks so 
wet as to prevent sufficient desiccation to produce coherent films. This latter 
reason also accounts for the greater abundance of deposition in summer, when we 
should expect river water to be comparatively free from earthy matter, as com¬ 
pared with autumn, when we know it to be foul with it. It points out also why 
such rivers as the Tay, the Humber, and the Thames, where estuaries abound 
with Sand-banks, should be particularly favourable for the collection of mud for 
embanking, warping, and in the formation of alluvial marshes, compared with 
others, which, bearing an equal quantity of mud alone, carry it out without 
detention far into the sea. 
Cupar , Fifes hire, 
Aug. 2, 1838. 
