To Charles Lyell   [16 December 1843]

Down. nr. Bromley Kent

Saturday

My dear Lyell

I have been looking over my huge bundle of notes & I find that the proposition that the Tosca was a diluvial mud is monstrous.—1 It is distinctly stratified in some parts & often shows differences in the upper & lower parts. It is not the last deposit; at least a grand formation of absolutely similar character, underlies in Banda Oriental a great formation of red sandstone & conglomerate. At the mouth of the Uruguay, we have the following section, in a vertical cliff. diag A

———

Limestone, often cellular &

crystalline or impure, with

numerous sea-shells.

———

seams of white sand

———

Limestone

———

30 feet of pale clay with

many layers of oysters

———

unideniable tosca or

pampæan mud

———ramme And near this place, I found tosca, resting on the limestone, (at A) with the bones of Mammalia.2 —. In fact we have everywhere proofs, that in the midst of a grand estuary formation of an intercalation of a pure marine one.—3 A little higher up the Uruguay, we have this section.— diag —————————

oddly variegated agate &

limestone

—————————–

coarse comglomerate

—————————–

gritstone with more or less

calcareous matter

—————————–

a thin bed of the ordinary red

tosca

—————————–

calcareuous gritstone &

limestone

———ramme This is the uniform diluvian mud.!4

In fact there appears to be an older & newer tosca. The upper tosca no doubt retains same character over wide spaces, though to the East in the granitic districts it gets very sandy & this upper tosca bed chiefly contains mammal bones.—5

Even at the Rio Negro, (D’Orbigny’s own country) I find in a cliff 200 ft high of various sandstones, two thin beds of perfectly characterized tosca.—6 I find that the conglomerate of pumice in sandstone in the Patagonian Tertiary is apo-cryphal.— I found merely great fragments & could not in the high overhanging cliffs find the actual bed, which in my notes, I regret.— I daresay I have given you more details, than you care to hear.

So farewell | Ever yours | C. Darwin

1
‘Tosca’ is the South American term for the nodules of sand and calcium carbonate imbedded in the red clayey mud of the Pampas formation (South America, pp. 76–7). Alcide d’Orbigny, who mentioned no tosca, had argued that the red clay of the Pampas had been washed in, following the Tertiary, by a single great catastrophic flood associated with the sudden elevation of the Andes (Orbigny 1835–47, vol. 3, pt 3: 72–3, 81–8, 253–5).
2
CD believed that the red clay and tosca formation he found beneath marine Tertiary deposits near Punta Gorda and the mouth of the Uruguay was identical to that of the Pampas formation (Journal of researches, p. 171). Believing that the Pampas deposit was due to a unique catastrophe Orbigny rejected CD’s assertion (Orbigny 1835–47, vol. 3, pt 3: 27 n., 40 n.). However, CD accepted the possibility of mineralogically similar deposits appearing in different periods (South America, pp. 91–3). Though subsequent research has indicated that the two formations are not identical, they were produced by similar volcanic and alluvial processes (Falconer 1937).
3
CD held that the Pampas clay had been deposited in an estuary during gradual elevation of the land (Journal of researches, p. 149).
4
Alcide d’Orbigny had stressed the unstratified character of the Pampas deposit as an argument against any gradual deposition (Orbigny 1835–47, vol. 3, pt 3: 73, 84).
5
Described in Journal of researches (pp. 95–7, 149–55, 180–2).
6
Described in South America (p. 108).

Manuscript Alterations and Comments

1.1 bundle] altered from ‘bundles’
1.5 At] over illeg
1.8 &] over comma
1.18 in the midst] ‘in’ over illeg
2.1 upper] above del ‘newer’
2.2 in the granitic districts 2.3] interl
2.3 upper tosca] interl in pencil
3.2 two] over illeg
3.3 is] before del ‘a’

Please cite as “DCP-LETT-724,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-724