[The section on Bahia Blanca starts on CUL-DAR32.62. The text is repeated here for clarity.]

Bahia Blanca

The northern shore of this bay is rather less than 30 miles along, in all this line I only found three v. notes !!! geological sections; the intervals being completely covered up by sand dunes. — The settlement or town is built on a slight elevation in an extensive plain; the rock is composed of horizontal strata of a soft spongey friable pale coloured argillaceous limestone 716, containing small pebbles of quartz & other extraneous matter. — Being there only a few hours I know nothing of its relation to the great plain: —

Punta Alta. projects into the bay & is formed of a mile of low cliffs. — it possesses great interest to the geologist from containing numerous organic remains. — The lowest bed extends horizontally for some miles at low water mark; it is a gravel of quartz pebbles (b) cemented by calcareous matter, is hard & made up of plates, which at first sight look like stratification. — Within in some

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places there are included curved plates of marl & clay. — This gravel contains great numbers of shells & fragments of bones. (Vide section) — On it rest a bed about 15 feet thick, of a red earthy clay. containing a few, small pebbles & few fragments of shells, it is very tough & hard difficult to be broken, though not hard or adhaesive. — it contained minute cavities resulting probably from the decomposition of vegetable matter: This substance is called by the Spaniards "Tosca", which term I will employ for brevity. —

The Tosca here only contained some most singular osseous plates, which will be mentioned presently: but at Monte Hermoso it contained great numbers of bones of quadrupeds. — On each end of (a) the cliff the Tosca is covered or passes itself insensibly into the same sort of cemented gravel, containing numerous shells. — It is probably by the protecting influence of this that the cliff yet remains; for it is evident that some miles of Tosca have has been washed from above the gravel. Covering the whole is a layer of diluvium & vegetable mould, the diluvium contains chiefly pebbles of quartz (c) & in siliceous sandstones or quartz rock, there are arranged in rows & of the size of an egg. —

Amon (It is to be remarked that the siliceous sandstone something resembles that which is found at Rio Negro). — Also in the diluvium there occurs pebbles of green phonolite & pumice. 810 — I imagine these came from the Andes, although distant about 500 miles; it may be objected, that the (a) Sierra di Ventana is only 45 miles distant & that the summit of highest hill has a crater-like appearance.

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(a) It is also called Monte "Hurtado",¹ or the separated mountain, referring I suppose to its isolated situation in the Pampas:
Is it the same as the Casuahati of Falkner? yes²

1833 (b) The Tosca gives me the idea of replacing the gravel, then passing into it. —

(c) These pebbles unquestionably are all owing both here, & on the coast to the Sierra Ventana. — (45? miles distant). — How came they here? by former streams or violent commotions. — The Anterior to the Tosca plain. — It is not very easily solved; but the latter case must have happened. — viz. anterior to Tosca plain. —

¹ See the mention of this name in the Beagle Diary, p. 365.

² See the mention of this name in the B. Blanca notebook, p. 2a. Falkner 1774.

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I think however that the ridge of chain is too sharp & the outline of hill not enough rounded for a volcanic mountain. — Also if it was covered by Pumice, the reflected light would be plainly visible, this was very clear when the Sierra was covered with snow: — The cliff is perpendicular to the sea: behind it is a small piece of tall land on which is a line sand dunes. & behind them a flat extensive plain. — The cemented gravel as before mentioned contains numerous organic remains: 1^st shells there are so numerous as in places, especially the upper bed, almost to compare them it; they appear to me to be exactly the same species which now exist on the beach: And it is to be especially remarked the proportional numbers of each species are about the same; the most abundant in both cases are Crepidula Voluta. Trochus & Venus 811: 812: 813; the rarest Pecten Fissurella. — 2^d: 2 Coralls. an encrusting Flustra & an Astraea. these both appear identical with what now exist: — 3^d a piece of wood converted into calcareous matter (?) 728 729: — 4^th the number of fragments of bones of quadrupeds is exceedingly great; — I think I could clearly trace 5 or 6 sorts. — the head of one very large animal 821 (with singular anterior cavity) has 4 large square. hollow molar teeth; perhaps it may be the Megalonyx: the lower jaw & one molar tooth of some smaller animal. I conjecture one of Edentata 822 & perhaps allied to the Armadilloes: the molar teeth of some large animal (a). (Rodentia?) 741 .... 744: bones of some smaller quadruped 758. 759. like deer: These bones

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(a) If this mere conjecture should be right; S. America would possess the largest living species of the Gnawers in the Cavia & a Capybara, by far the largest fossil one. — in the fossil

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are generally white, soft & friable; thence it is difficult to cut out of the gravel perfect specimens; they have entirely lost their animal matter, neither giving any smell or blackening under the blowpipe. — This softness is perhaps owing to their having remained for some time at the bottom of the sea. — which is shown by Balani adhaering to them. 782 — This mixture of such quantities of bones of land animals with shells, must be explained by supposing a body of water sweeping over the plain & bringing with them the bones strewed on the surface & the living animals; & that in this place these & the pebbles were deposited together; the finer particles being carried on by the stream. — the strata like form of beds. sometimes curved & dipping to different quarters must be explained by the same manner: — The upper gravel only differs from the lower in containing fewer bones. (we may conjecture the first inundation swept the plains clear) & that the action of water is evident in breaks & furrows on the Tosca. — This latter I imagine to have been deposited under similar circumstances, but in calmer water. — it is impossible to behold it, without immediately imagining ? it is the mass of earth which a debacle tearing across the country would deposit. — At Punta Alta, the only organic remain I found in the Tosca (excepting mere particles of shells) was a most singular one: it consisted in an extent of about 3 feet, by

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7 2 covered with thick osseous. polygonal plate; forming together a tessellated work: it resembles the case of Armadillo on a grand scale 735 ... 739 & 807: 808: these plates were double, on an interval of few inches between them. (a) — With it was only a fragment of joint of extremity. — At present the case of the dead Armadillos are oftener found separate from the body. then connected with any part. — In this case the envelope of the great animal would easily be carried by the water, & by the pressure of Tosca would be doubled up as described. — It is stated. that "recent observations" show the Megatherium had such an envelope; It certainly is probable it belongs to some of the animals the bones of which are so abundant in the gravel. — In connection with the Megatherium I may mention a curious fact. — It is a common report in all these parts of S America that there exists in Paraguay, an animal larger than a bullock, & which goes by the name of "gran bestia"

The Commandante at the Fort. states that he many years ago saw a young one, when in [Tanagung]; that it had great claws & snout, like Tapir. (b) (He added also that it is carnivorous; having only seen a young one this must be conjectural). Now these are the very words with which Cuvier describes the probable figure of the Megatherium, the fossil bones of which are well known to come from Buenos Ayres & Paraguay. — If no credit is given to the actual existence of the "gran bestia" we must suppose it is either traditional or that it is a cer a report

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(a) Falkoner mentions having found hexagonal plates similar to these on the banks of the Carcarana. — & compares them to an armadillo. — P 55

(b) Upon talking with this man, it was evident he referred to a Tapir, which exists to the North of his native place in Entre Rios.

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arising from the occurrence of very perfect skeletons. — the resemblance is too striking to be attributed to mere chance ! — — The sea is now weaving away & destroying the cliffs; whilst on other places the land is gaining by what is heaped up by the tides. — A sandy flat, which is covered at high water, is generally bounded by a steeper beach, formed of shells & pebbles. — Behind this, for about two miles within the country, there is succession of parallel sand hillock, with flat intervening vallies. (c) — These dunes rest (I found a section) on shells & pebbles & the upper parts are formed of drifted sand. which is bound together by the long roots of grasses. —

[sketch, ink over pencil] y Tosca plain m k x k x k a b high water

This process is now going on: a bank (b) is often formed in the flat, which by the action of gales &c is formed into a beach as at (a). The intervening space soon becomes filled up & then corresponds to (x). And finally behin by the action of wind dunes (k) are heaped together. — Within this present formation there is a wide horizontal plain: I dug a hole 3 feet deep in it (b): in all the characters of the it is composed of the same Tosca as we have seen at Punta Alta. (a) — It is probable this bed runs for a long distance: at M Hermosa (as presently will be shown) we find it, although distant in

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(a) It also contains few small fragments of shells:

(b) The hole was dug in a plain about 3 miles Easterly of Punta Alta

(c) It is curious how very perfectly. the dry nature of this sandy soil preserves insects, when we first arrived there at B. Blanca no insects were yet moving about: the spring was not far enough advanced, on the surface however I found the remains of the greater number of Heteromerous insects & Lamellicora. in such a perfect state that I had intended keeping them: it is probable these had been exposed to all the changes of the winters climate: How easily would these have been preserved in any geological strata. — the insects found at Aix are such as inhabit dry localities in Everyone must have observed how uncommon it is in most situations to see any number of the remains of Coleoptera:

(d) is bank (b) formed at time when ebb & flow meets

(e) on the high level plain of Limestone N & NE of the Fort covered by sandy dry soil. — there are immense numbers of 3 or 4 species of Land shells. — for several leagues. every square inch had its shell; it was like a sea-beach. — I never saw anything at all like it before. — They were nearly all dead shells 1514. specimens (1514) were all dead excepting one. — I partly account for numbers by preservation for many years from dry nature of soil. & absence of luxuriant vegetation. — Where insects have been found in geological formations have Land shells likewise been there. —

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a direct line 18 miles from Punta Alta. In all three places, on the surface there occurs the singular presence of Pumice pebbles. — I imagine this Tosca plain extends beneath the dunes; since upon digging a hole in any of the vallies to some depth (b) water in a sh some time will collect. — The Tosca plain is itself bounded by a chain of dunes. (z), which must be much more ancient as old shells do not occur on the surface: Penetrating still further into the country we meet an undulating sandy plain of somewhat greater elevation. —

As far as I am able to understand the formation of the country; it is as follows. — After the deposit of Tosca & cemented gravel, formed by the entrance of into the sea of debacles having flowed over the land: (— things remained quiet long enough for the chain of dunes ? ) (a) which extends behind Punta Alta & the plain (m) to be formed:) And whole country was elevated. — At this period or later the diluvium was deposited, which I have said probably owes its origin to a flood coming from the W in direction of the Andes. — I do not know whether the occurrence of Pumice in this diluvium (B) & on the plains points out any closest connection with the elevation subsequent to the elevat of the land: The degree of elevation probably was not great. as it only served to lift deposits, only a few feet

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(a) As I have no evidence that these dunes rest on a beach of shells: it is more probable they were formed after the elevation of the plain & previously to the chain of to hillocks now forming (b) near the coast about from 8 to 12 feet. — to this cause, the water being prevented penetrating downward [sketch], a much greater degree of fertility adorns these sand hillocks than would have been expected: —

(B) Falkner mentions that he saw a cloud of volcanic ashes, which were blown from near Mendoza to the Sierra de Ventana, where he then was. P 51

Some of the modern sand-dunes at B. Blanca are between 80 & 90 feet high 50 & 60 ft. — Capt Fitz R (?)

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above the level of the ocean, which were evidently formed near to the land. — Subsequent to this elevation, the present order of things commenced, the sea has continued wearing away some points & in others heaping up bank, thus wearing creating a line of coast. best adapted for its own motions. — It is certain that this period is a long one, from the breadth & number of the parallel dunes. — Having no data to calculate the time in which one is formed. it is impossible even to conjecture the absolute time: —

I will now describe the section of cliff at Monte Hermoso. situated at the entrance of B Blanca: & like Punta Alta forming a head-land, which the sea is continually washing away: (a)

The formation may be divided into three parallel beds: — these to the eye are horizontal (: ? but in the coast section, running NW by W, it is manifest the lower beds thicken towards the point: All Magnetic Var: 14 E: therefore perhaps the beds dip within some points SE by E & a range NE by N & SW by W?). — The upper bed is a dark sand forming a soft sandstone, about 20? feet thick; there are in it, false lines of stratification as at Punta Alta; & in places quartz pebbles: the upper parts have been much weathered & connected into sand & broken by the action of weather water: — I have before mentioned the occurrence of pumice pebbles: I should think the answer. to, that is formed at the same time as, the upper gravel at P: Alta. —

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(a) Page 50. I have mentioned that the soundings are generally greater, opposite to the cliffs; at. M: hermosa within 1/2 a mile there is 5 fathoms water; from this point for some miles out to sea it gradually deepens a fathom per mile. — The bottom consists of very tenacious clay, so as to make it very difficult to weigh anchor: (I imagine this clay is formed by the action of water on the Tosca. — where the sand is washed away, but the argillaceous matter remains. from its adhesiveness?. It is certain it is a present formation from the extreme regularity of the shoal. —

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It is separated from the inferior beds by narrow line, about 6 inches thick of much more compact & hard sandstone:

Beneath this is the Tosca. which agree in all its mineralogical characters with that at P Alta: It may be divided into two beds. the upper pale coloured about 12 more feet thick 769, the lower dark red about 15 more feet: — In neither of them was there any lime: they agree likewise in containing numerous fragments of bones, but no marine remains. The lower red bed is remarkable however (b) in containing at its base horizontal layers & nodules of concretionary marl. — marl, compact, fracture slightly conchoidal, coloured oink & dendritic with Manganese. 770: 771 804 — In places where the nodules were most abundant, it reminded me of the flints in chalk: — I was surprised to see so much chemical action as these concretions prove, in so mechanical a formation. — — The bones in their nature were singularly different from those at P Alta; in the one case they had been immediately enveloped in the Tosca, in other exposed to the action of the water:

Here the bones were very hard & of great specific gravity, their surfaces polished & blackened externally; in the smaller ones they, from this cause resembled jet. 772 — I could perceive traces of 4 or 5 distinct animals; two of which certainly belonged to the Rodentia. 799: 800 One must have been

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(a) In contact with the marly concretions. the Tosca is more like pure clay; the surface also is often in rays:

(b) At Punta Alta in the gravel. just beneath the Tosca. I have noticed curved plate of argillaceous & marly substance: proving helping to prove the general resemblance of two formations. —

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allied to the Agouti; the tarsi & metatarsi belong to an animal 801 less than the present common inhabitant, Cavia patagonica. (b) the Agoutis are all proper to S. America; & none have hitherto been found in a fossil state: — To conclude with the organic remains. — I have shown that some of the bones probably belong to the Edentata. & that the osseous plates are supposed to belong to the Megatherium. — The remains of this animal have (a) been always described as being in superficial gravels & caverns. — or as it is sometimes called diluvial formations. — Now it appears to me, that the beds, which I have described, do not come in this class; from the following reasons. — 1^st the occurrence of a bed of matter, probably transported from great distance. & which itself would more justly correspond to such a class: 2^d the large extent of beds, which both in their characters & superposition are not widely different. 3^d the chemical formation of the concretionary marl: — I should think these beds deserved the name of a regular formation, as much as the Tertiary. — That they are comparatively modern is certain from the similarity of shells & the features of the country not being greatly different. — Some geologists have been surprised that the extinction of land-animals, has not occurred, without

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(a) In the newspaper accounts, the Megatherium lately presented by M^r Parish¹ to the Geological society Novm. 20^th,[1832] is stated to have been found in the mud in bed of the river Salado. — Upon examining M^r Oakley, who procured it for M^r Parish, it seems the river flows through cliffs of the Tosca. & which doubtless is identical with that of Bahia Blanca & Buenos Ayres (V b) from the his stating the occurrence of layers of concretionary marl & M^r Oakley clearly recollects that one of the scapulas was imbedded in th a mass of Tosca: also that large masses of cliff did were often fall into bed of the river. — That the bones were close to the bank is shown by the statement Gauchos that they by their lassos dragged some of the pieces out. — I think all this demonstrates that this perfect skeleton is in the same geological position as those I found: —

(b) It is interesting to observe that this tribe of animals. the Agoutis which are now peculiar to the Americas, should in the epoch, when the Megatherium flourished, also be present. — Showing that with the extinction of one genus, that of others did not follow.

¹ Woodbine Parish (1796-1882), the British consular representative. He is mentioned various times in the Beagle notebooks and the Geological diary, see here. See Darwin to Henslow 11 April 1833. The bones were described in Clift 1835. Oakley was the agent of Woodbine Parish.

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destroying the inhabitants of the sea; this would seem to be a case in point. —

Upon looking at a chart of Bahia Blanca, one would suppose (a) from the number of banks that it was the embouchure of some large river. These banks are either composed of sand such as the outer ones & are probably heaped up, where the tides meet; or of soft adhesive mud. covered with rushes & overflowed by high water. — I can hardly attribute these to being deposited as is commonly the case; for I do not see where the water is to obtain so much matter; but rather to the general elevation of the ground, which has lifted the bottom of the old sea nearly to a level with the surface of the present. — & subsequently eat into channels, : or covered with mud or sand. —

I have been this particular in describing these beds, in which the organic remains occurred. — for the comparison of formation in different parts of the world which contain animals of equal grade in the chain of nature, seems at present to be much wanted in Geology. —

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(a) These mud banks are very soft, will not support a horse. are called Cangrejales; from Cangrejo a crab. (I suppose) from being inhabited by so many of those latter animals, which make holes. —

This explanation will apply to all the extensive mud banks or low island between B. Blanca & P. Raza. — the map of which would resemble an embouchure of a great river. —