6 
ON DEPOSITIONS OF MUD IN THE TAY. 
a remarkable clearness and transparency takes place .in the colour of the water; 
all muddiness disappears, and the bottom may be distinctly seen at depths far 
beyond those at which it could a few hours before be observed. This is the 
period selected for bathing by amateurs, who have noticed and are aware of 
these facts. At this time, as may be supposed, the salt part of the tide is strongest, 
being the true oceanic tide, which insinuates itself like a wedge beneath the 
ponded fresh-water, which it elevates on the surface,* just beginning to mingle 
with the fresh water of the river, as both flow towards the sea. On the 
more rapid rush of waters taking place towards half-tide, when the sand-banks 
begin to appear above the surface, a slight degree of turbidity ensues, chiefly, 
near the bottom, the surface remaining pure and transparent. Immediately 
on the turn of the tide from ebb to flood, appearances become entirely altered. 
The surface of the advancing waters cease, on the side setting up the river, to be 
transparent as formerly, but appear covered over with Aims of some opaque or 
oleaginous matter. So uniformly and universally had this become matter of 
observation, that I found that around Dundee the school-boys, who amused 
themselves by fishing or wading about the mud banks at low water, took the 
discoloration as the first evidence of the turn of the tide, and the signal to retreat 
before an increase of depth sufficient to occasion alarm could be observed. Nor 
was this confined to Dundee, or the debouchure of the river, or its confines ; as 
high as the salt water ascended the same phenomena took place, and continued 
through the periods of flood. Now it was that the deposit of silt occurred, and 
that a tumbler full of water in this condition, exhibited precipitation, the earthy 
matter falling to the bottom not as a uniform precipitate, but in thin, floccular, 
minute flakes of discoloured snow. As the tide approaches flood, and begins to 
fall away again, the same appearances as above-described once more ensue, and 
the water purifies itself as the tide falls back, till a rise exhibits it defiled on the 
surface as before. 
The idea that the muddy sediment was an oceanic and not a fluviatile deposit, 
seemed extraordinary ; yet supported by so many appearances, that on examining 
the water, the following theory seemed to reconcile all difficulties without any 
striking or strained hypothesis. 
The estuary of the Tay, for a space of twenty miles in length above the sea, 
and from one to four in breadth, abounds with banks of sharp silicious Sand, 
which become dry about half tide. These are perfectly permeable, and charged 
at all times with water, as may be seen when a hole is dug below the level of 
the tide, which is immediately filled. Their texture is such, however, that 
* See Dr. Fleming on Salt and Fresh-water tides in the Tay, in Philosophical Transactions, 
Yol. VIII., 1818. 
