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