On the northern bank of the great estuary of the Rio Plata, near Maldonado,
I found at the head of a lake, sometimes brackish but generally containing
fresh water, a bed of muddy clay, six feet in thickness, with numerous
shells of species still existing in the Plata, namely, the Azara labiata,
d'Orbigny, fragments of Mytilus eduliformis, d'Orbigny, Paludestrina
Isabellei, d'Orbigny, and the Solen Caribaeus, Lam., which last was
embedded vertically in the position in which it had lived. These shells lie
at the height of only two feet above the lake, nor would they have been
worth mentioning, except in connection with analogous facts.

At Monte Video, I noticed near the town, and along the base of the mount,
beds of a living Mytilus, raised some feet above the surface of the Plata:
in a similar bed, at a height from thirteen to sixteen feet, M. Isabelle
collected eight species, which, according to M. d'Orbigny, now live at the
mouth of the estuary. ("Voyage dans l'Amerique Merid.: Part. Geolog." page
21.) At Colonia del Sacramiento, further westward, I observed at the height
of about fifteen feet above the river, there of quite fresh water, a small
bed of the same Mytilus, which lives in brackish water at Monte Video. Near
the mouth of Uruguay, and for at least thirty-five miles northward, there
are at intervals large sandy tracts, extending several miles from the banks
of the river, but not raised much above its level, abounding with small
bivalves, which occur in such numbers that at the Agraciado they are sifted
and burnt for lime. Those which I examined near the A. S. Juan were much
worn: they consisted of Mactra Isabellei, d'Orbigny, mingled with few of
Venus sinuosa, Lam., both inhabiting, as I am informed by M. d'Orbigny,
brackish water at the mouth of the Plata, nearly or quite as salt as the
open sea. The loose sand, in which these shells are packed, is heaped into
low, straight, long lines of dunes, like those left by the sea at the head
of many bays. M. d'Orbigny has described an analogous phenomenon on a