Chapter 1

ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.

Upraised shells of La Plata.
Bahia Blanca, Sand-dunes and Pumice-pebbles.
Step-formed plains of Patagonia, with upraised Shells.
Terrace-bounded Valley of Santa Cruz, formerly a Sea-strait.
Upraised shells of Tierra del Fuego.
Length and breadth of the elevated area.
Equability of the movements, as shown by the similar heights of the plains.
Slowness of the elevatory process.
Mode of formation of the step-formed plains.
Summary.
Great Shingle Formation of Patagonia; its extent, origin, and distribution.
Formation of sea-cliffs.

In the following Volume, which treats of the geology of South America, and almost exclusively of the parts southward of the Tropic of Capricorn, I have arranged the chapters according to the age of the deposits, occasionally departing from this order, for the sake of geographical simplicity.

The elevation of the land within the recent period, and the modifications of its surface through the action of the sea (to which subjects I paid particular attention) will be first discussed; I will then pass on to the tertiary deposits, and afterwards to the older rocks. Only those districts and sections will be described in detail which appear to me to deserve some particular attention; and I will, at the end of each chapter, give a summary of the results. We will commence with the proofs of the upheaval of the eastern coast of the continent, from the Rio Plata southward; and, in the Second Chapter, follow up the same subject along the shores of Chile and Peru.

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,