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1833 Bahia. Blanca. (appendix. —

Having revisited P. Alta, seeing the neighbouring country. my opinion respecting its geology is completely altered (a) — & renders superfluous the greater part of the following pages. — The P. Alta bed is not coeval with the great Tosca formation: this is clear from the Tosca, containing shells, from the presence of Quartz pebbles, from & those of Pumice & (chiefly) rolled masses of Tosca rock. Its present position is owing to the last elevation, which this country has undergone. — for I cannot quite think, it has been formed under present circumstances; there is no doubt that such a bed of gravel might easily be thrown up; but these alternations of Tosca. with horizontal variations in colour & arrangement of pebbles do require some little depth of water; many other phenomena. require are best explained by small modern upheaval: — mud banks: — V following pages compare Tosca plain. with fragments of shell behind coast sand dunes (b) (this indubitably must have been formed beneath water & no change of tides &c &c would be sufficient to produce it): the higher sand-dunes, without shells, behind plain: & lastly the basin, in which the Fort stands, this is composed of swamps & marshes, & lines of low sand dunes, with small Turbo's (such as now exist on mud bank); all without any appearance of the, & the detritous plain of which the Town stands, it is composed of Tosca tock. in a whitish marl with fragments of quartz. — now the great Tosca formation never contains any pebbles; this is level & about 30

73 verso

(a) I am not surprised at my mistake

(b) The Pumice, which is in the gravel at P Alta & Mt: Hermoso. & scattered over the low impure Tosca plains; must have been deposited, when the country was beneath the sea, previous to last upheaval. — We have however plenty of instances of Pumice being now brought from the Andes. — On the banks of the R. Negro I saw much & the same officers in the small Schooner. found scoriæ &c in the R Chupat

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1833 B. Blanca

feet above the swamps; therefore posterior to true Tosca & formed under water. — The basin is surrounded by cliffs, (a) presicely such as wood surround a bay with a sea. & such I believe it was when. P. Alta was formed. — With respect to the age of M: Hermoso, I am doubtful: the absence of shells & pebbles in Tosca, the presence of Tosca rock concretions, the grouping of the bones widely separates this bed from P. Alta. — I do not however think it contemporaneous with the great Tosca formation chiefly from the soft sandstone with quartz pebbles; — perhaps it may be coeval with the Tosca of B. Oriental & that at Bajada, which overlies the Limestone & which does not generally contain so much calcareous Tosca rock. & has so many bones. — From the scattered rolled state of the bones at P. Alta. & from one small pieces (1541) 1541; like those jet ones of M: Hermoso. I have no doubt many of 5 bones owe their origin to such beds as those of M: Hermoso. — All the bones however cannot do so, for instance, the most perfect skeleton (1405) 1405 (X) (L), must have been washed entire to near the place where found. & therefore a very modern animal coeval with present shells & posterior to great Tosca formation. —

The chief thing proved by this place, is an upheaval posterior to great Tosca plain; & to the coevality of certain animals at. M: Hermoso & the extreme newness lateness of existence of Animal (1405). —

Generally the P. Alta bones can prove nothing; nor indeed the M: Hermoso with reference to other formations. —

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(a) Also the sand dunes above the edge of cliff on South side of Basin. —

(b) This skeleton was not in the gravel, like the other bones, but held together in the agglutinated sand it was 1/2 imbedded in loose sand at low water: the different parts of skeleton were going nearly in their proper part of position; I suppose this agglutinated sand is part of the gravel formation. — & the whole mass broke from some part of the cliff or bed & left where it now stands

This animal in all probability lived with those which now exist: for this last & small elevation must have been very recent & previous to this the animals body was washed into a shallow sea. —

It may strongly be suspected that the large piece of Megatherium hide, with some bones, could not be owing to the action of the sea on previous cliffs.

(c) I do not think it is quite certain, but that the lower beds, with quartz granules, may be original formation, differing from the superior beds, as at S. Cruz. —

Black Bone R. N p. 68¹

¹ Darwin wrote in his R.N. or Red notebook "Fossil bones black as if from peat. — yet cetaceous bones so likewise [of miocene period]. — Mem Bahia blanca P. 204 Vol III. Lyell".