As the Thames is itself the product and union of all its vassal streams,
an "incarnation" of all the rest, so in its bed it holds all the shells
collected from all its tributaries. Different tribes of shells live in
different waters. Some love the "full-fed river winding slow," some the
swift and crystal chalk-stream. Some only flourish just over the spots
where the springs come bubbling up from the inner cisterns of earth, and
breathe, as it were, the freshness of these untainted waters; others love
the rich, fat mud, others the sides of wearings and piles, others the
river-jungles where the course is choked with weeds. But come what may, or
flourish where they please, the empty shells are in time rolled down from
trout-stream and chalk-stream, fountain and rill, mill-pool and ditch,
cress-bed and water-cut, from the springs of the Cotswolds, the Chilterns,
the downs, from the valleys of Berkshire, Buckinghamshire, Surrey,
Gloucester, Oxford, and Essex, into the Thames. Once there the river makes
shell collections on its own account, sorting them out from everything
else except a bed of fine sand and gravel, in which they lie like birds'
eggs in bran in a boy's cabinet, ready for who will to pick them up or
sift them out of it. These shell collections are made in the time of
winter floods, though how they are made or why the shells should all
remain together, while sticks, stones, and other rubbish are carried away,
it is impossible to say. They are laid on smooth points of land round
which the waters flow in shallow ripples. Across the river it is always
deep, swift, and dark, though the sandbanks come in places near the
surface, and in the shallows grow water-crowfoot, with waving green hair
under water, and white stems above it. The clean and shining sand shelves
down to the water's edge, and continues below the surface. Here are living
shells, or shells with living fish in them. In the bright water lie
hundreds of the shells of the fresh-water mussels, the bearers of pearls
sometimes, and always lined with that of which pearls are made, the
lustrous nacre. The mealy masses of dry sand beyond the river's lip are