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3. On the THICKNESS of the PAMPEAN FORMATION, near BUENOS AYRES.

By CHARLES DARWIN, Esq., M.A., F.R.S., F.G.S., &c.¹

M. SOURDEAUX² and J. Coghlan,³ Esq., C.E., have had the kindness to send me, through E. B. Webb,⁴ Esq., C.E., some excellent sections of, and specimens from, two artesian wells lately made at Buenos Ayres. I beg permission to present these specimens to the Geological Society, as they would be of considerable service to any one investigating the geology of that country. The Pampean formation is in several respects so interesting, from containing an extraordinary number of the remains of various extinct Mammifers, such as Megatherium, Mylodon, Mastodon, Toxodon, &c., and from its great extent, stretching in a north and south line for at least 750 geographical miles, and covering an area fully equal to that of France, that, as it appears to me, a record ought to be preserved of these borings. Southward, at the Rio Colorado, the Pampean formation meets the great Tertiary formation of Patagonia; and northward, at Sta. Fé Bajada, it overlies this same formation with its several extinct shells.

In the central region near Buenos Ayres no natural section shows its thickness; but, by the borings there made in two artesian wells (figs. 1 & 2), the Pampean mud, with Tosca-rock, is seen to extend

¹ This was reprinted in Geological Observations 2d ed., pp. 363-7.

² Adolfo Sourdeaux, engineer who bored artesian wells in Buenos Aires.

³ John Coghlan (1824-1890), government engineer and member of the Board of Public Works in Buenos Aires.

⁴ Edward Brainerd Webb (1820-1879), civil engineer working in London.

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downwards from the level of the Rio Plata to a depth of 61 feet, and to this must be added 55 feet above the level of the river. These argillaceous beds overlie coarse sand, containing the Azara labiata (a shell characteristic of the Pampean formation), and attaining a thickness of about 93 feet*. So that the entire thickness

Fig. 1.—Comparative Sections of the Artesian Wells of Barracas and Buenos Ayres. (Distance 3¾ miles).

			Thickness at Barracas. Feet.		Thickness at Buenos Ayres. Feet.
a.	Clays and Tosca........................................		...	.........	57
b.	Sand.......................................................		13	.........	51
c.	Very sandy clay........................................	}	47	.........	52
d.	Dark-blue plastic clay................................
e.	Tosca with calcareous nodules
f.	Yellow sands, very fine and fluid.................		94	.........	45
g.	Green sands.............................................		66	.........	62
h.	Tertiary clay and sandstone (for details see fig. 2)		34	.........	33
k1.	Hard sandstone at the bottom of the Barracas Well.......................................................		4½
k2.	Very calcareous red clay, becoming more marly beneath; bored through to a depth of............		...	.........	225

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* The following extract from the Report of the borers relates to this bed:—"The bed of yellow, fluid sands between 18^m .60 and 47^m .20 below the ground

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of the great estuarine or Pampean formation near Buenos Ayres is nearly 210 feet.

Fig. 2.—Detailed Section of the Artesian Well at Barracas.

			Thickness in metres.
	a.	Sand...................................................	4.33
	b.	Very arenaceous clay.........................	8.02
	c. {	Fine clay.............................................	1.05
		Blue plastic clay.................................	2.90
	d.	Tosca with calcareous nodules..........	2.30
	e.	Yellow sand, very fine and fluid, with quartz-pebbles and fluviatile shells..................................	28.60
	f.	Green clay, more or less plastic and calcareous, with iron-pyrites, marine shells, and nodules of lithographic stone.........................	20.30
	g.	Green sand, with shells and quartz-pebbles..............................	.80
	h.	Shelly limestone.........................................................................	.45
	i.	Calcareous clay....................................................................	2.00
	k.	Shelly sandstone...................................................................	.25
	l.	Green arenaceous clay.............................................................	2.00
	m.	Shelly sandstone.....................................................................	.30
	n.	Speckled sand..........................................................................	.70
	o.	Very compact arenaceous clay..................................................	2.25
	p.	Coarse sandstone....................................................................	1.40
	q.	Green sand, very fine and fluid, with quartz-pebbles and shells...	2.35

This formation rests on various marine beds of indurated green clay, sand with corals, sandstone, and limestone, altogether 107 feet in thickness. These beds contain fragments of the great Ostrea Patagonica, O. Alvarezii (?), Pecten Paranensis, and other shells, apparently the same (but they have not been rigorously compared) with those enumerated by M. A. d'Orbigny and by myself as found at Sta. Fé Bajada, as well as at various points on the coast of Patagonia. The already enormous continuous extension of the Patagonian Tertiary formation is thus largely increased. Beneath these beds a mass of red calcareous clay, becoming in the lower part more and more marly, containing layers of sand, and of the thickness of 213 feet, was bored through to a depth of 470 feet from the level of

contains a subterranean ascending current, the level of which has not varied by a centimetre for three years. The level is 0^m.60 (2 feet over the level of the wells at Barracas). This bed ('napa') is powerfully absorbent. At 68^m.30 a second subterranean current ('overflowing') was met, which rose one foot over the surface of the ground at Barracas. The discharge was about 50 pipes daily, but the water was salt and undrinkable. At 73^m.30 was found a third subterranean current ('overflowing'), which reached with difficulty the level of the ground. The discharge might be calculated at 100 pipes daily. The water was very salt, and absorbed that of the first overflowing current. The great spring was met with at 77^m.65."

As regards the quality and abundance of the water, Mr. Coghlan remarks that "The quantity of water discharged per hour through a tube of about 4¼ inches in diameter, at a level of 6 feet above high-water mark, was 2658 gallons. Its temperature was 21° Cent., and it had a slightly disagreeable taste, from its being impregnated with salts of lime and magnesia and a small quantity of sulphuretted hydrogen."

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the Rio Plata. This lower mass contained no fossils, and its age is of course unknown*; but, I may add, that I saw at two points in Western Banda Oriental, beneath the marine tertiary strata, beds of red clay with marly concretions, which, from their mineralogical resemblance to the overlying Pampean formation, seemed to indicate that at an ancient period the Rio Plata had deposited an estuarine formation, subsequently covered by the marine tertiary beds, and these by the more modern estuarine formation, with its remains of numerous gigantic mammalia; and that, finally, the whole had been elevated into the present plains of the Pampas.

* It was supposed by Dr. Burmeister¹ to be Silurian.

¹ Karl Hermann Konrad Burmeister (1807-1892), German zoologist, geologist and Director of the Museo Nacional in Buenos Aires, 1861-1880.

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