第6章 ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AME

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, Ihave 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, 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 greater scale, near San Pedro on the river Parana, where he found widely extended beds and hillocks of sand, with vast numbers of the Azara labiata, at the height of nearly 100 feet (English) above the surface of that river.

(Ibid page 43.) The Azara inhabits brackish water, and is not known to be found nearer to San Pedro than Buenos Ayres, distant above a hundred miles in a straight line.Nearer Buenos Ayres, on the road from that place to San Isidro, there are extensive beds, as I am informed by Sir Woodbine Parish, of the Azara labiata, lying at about forty feet above the level of the river, and distant between two and three miles from it.("Buenos Ayres"etc.by Sir Woodbine Parish page 168.) These shells are always found on the highest banks in the district: they are embedded in a stratified earthy mass, precisely like that of the great Pampean deposit hereafter to be described.In one collection of these shells, there were some valves of the Venus sinuosa, Lam., the same species found with the Mactra on the banks of the Uruguay.South of Buenos Ayres, near Ensenada, there are other beds of the Azara, some of which seem to have been embedded in yellowish, calcareous, semi-crystalline matter; and Sir W.Parish has given me from the banks of the Arroyo del Tristan, situated in this same neighbourhood, at the distance of about a league from the Plata, a specimen of a pale-reddish, calcereo-argillaceous stone (precisely like parts of the Pampean deposit the importance of which fact will be referred to in a succeeding chapter), abounding with shells of an Azara, much worn, but which in general form and appearance closely resemble, and are probably identical with, the A.labiata.Besides these shells, cellular, highly crystalline rock, formed of the casts of small bivalves, is found near Ensenada; and likewise beds of sea-shells, which from their appearance appear to have lain on the surface.Sir W.Parish has given me some of these shells, and M.d'Orbigny pronounces them to be:--1.Buccinanops globulosum, d'Orbigny.

2.Olivancillaria auricularia, d'Orbigny.

3.Venus flexuosa, Lam.

4.Cytheraea (imperfect).

5.Mactra Isabellei, d'Orbigny.

6.Ostrea pulchella, d'Orbigny.

Besides these, Sir W.Parish procured ("Buenos Ayres" etc.by Sir W.Parish page 168.) (as named by Mr.G.B.Sowerby) the following shells:--7.Voluta colocynthis.

8.Voluta angulata.

9.Buccinum (not spec.?).