Hennessy — On Floatation of Sand by the Rising Tide. 155 
on the surface of a large basin of water, and after a few hours they 
were found grouped in parallel parcels, varying in their contents from 
two to seven needles. They continued to float for more than five 
days, and their sinking was evidently due to the progress of oxidation, 
which destroyed their polish, together with their repulsive action on 
the liquid. I have floated small flat pebbles, similar in size and ap- 
pearance to the largest of those observed floating on I^ewport river, for 
more than six days, while fragments of shells, and thin pieces of slate 
as broad as a sixpenny piece, have continued to float much longer. 
These little bodies occasionally sank from the gradual absorption of 
water, but much more frequently from some accidental motion of the 
vessel containing the liquid. 
It is manifest that the floatation of sand in a tidal estuary, as in 
the instance I have seen, can occur only under favourable conditions. 
The shores must be very gently inclined, the air perfectly calm, and 
the weather dry and warm. Under these circumstances thin cakes or 
sheets of sand may not only be uplifted by the' water, but if the tide 
flows rapidly they may continue afloat sufS.ciently long to allow many 
of them to be drifted far from their original place up to the higher 
limit of the brackish water. In this way fragments of marine shells 
and exuvise might become mingled with those belonging to fresh water. 
The conditions favourable for sand floatation must exist during calm 
weather in a very high degree of perfection on the sandy shores of 
tidal rivers in tropical and subtropical districts of the earth. As this 
phenomenon can take place only with the rising tide, and never with 
the falling tide, the result must generally be favourable to the trans- 
port of sand and marine debris in the direction of the flow of flood tide ; 
and this may sometimes hold good along a coast as well as on the shores 
of a tidal estuary. Greologists, as far as I am aware, have not hitherto 
noticed this phenomenon in connexion with the formation of stratified 
deposits by the agency of tides and rivers, although they have paid 
great attention to the influence of the molecular resistance of water 
to the sinking of very minute solid substances, with the view of 
explaining the wide surface over which matter held in suspension 
by water may be spread when ultimately deposited over the sea 
bottom. 
* Since this paper was written I have been informed by a lady, tbat she ob- 
served similar phenomena during a former summer, close to the sandy seashore at 
Youghal. - 
