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1835 Pampas 11

narrow, but from the time taken to cross it must [generally] be from 6 to 10 miles wide; now this

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the Portillo, the road turns North to Mendoza & follows this tr level tract. — The bottom is composed of an impure reddish clay with calcareous concretions (tosca) fine white sand with lines of minute pebbles in a fine white aluminous powder; these are horizontally stratified & alternate, from one section I believe such matter overlies coarse shingle. — On the surface in many place there are saline efflorescences, especially in parts near water. — About half way between Estacado & Luxan, I was enabled to examine the escarpement on the right hand. — It stretched in a continuous line N & S along the country; is perhaps 15 miles from the base of Cordilleras, is about 80 ft high, is composed of perfectly rounded large pebbles. 2646 (many as big as a mans head) of porphyries

these are white washed or cemented by a quite white

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Section of extreme Pampas, bordering on Eastern foot of Andes South of Mendoza — showing Andes as sloping platform. —

Basin with fine sedimentary depositions. — double escarpement

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by Miers. — The height of superior edge of Talus is estimated; the rise in such places is most deceptive & at first never appears half the reality. —

I take M^r Miers measurements in preference to Don Felipe Bauza,¹ because the former makes St Jago agree with the greater number of observers. — Capt. King. M. Gay &c &c Bauza gives St Jago 2620 in place of 17-1800 ft

Mendoza 4474!

¹ Felipe Bauza was a Spanish officer and geographer. Darwin probably found his information in Humboldt or Caldcleugh where he is frequently cited.

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1835 Pampas — Mendoza 12

friable aluminous substance. — The face of the cliff is tolerably steep, is surmounted by level plain of shingle (a), on which a little way further back is other & lower line of escarpement of another plain. — To the North of Mendoza, occasionally traces of a similar escarpement may be observed; on a sandy plain, there are also little water-worn hillocks of the sandy aluminous substances horizontally stratified. — Hence we have a band of low country parallel to the Andes, & which corresponds to the chains of basins in Chili. —

In the whole line of Cordilleras on at foot of the East slope there are many lakes & salinas; also on the West side, where there are plains to the North of Chiloe there are several large lakes. — All these facts must depend, on a subsidence or deficiency of elevation consequent to the upthrow of the grand chain. —

Are the fine sedimentary deposits of Mendoza lacustrine? on the birders. We know in the N. American lakes there are successive step-like terraces. — If such is the case, we can hardly avoid believing, that they must originally have been possessed by the sea, & subsequent to its retreat, the

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(a) Dry & small vallies cut through these escarpements & enter on the basin, precisely in the same manner as in Patagonia

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1835 Pampas Mendoza 13

salt water was replaced by fresh. —

I observed one fact which, if it was repeatedly verified would be curious. In the mouth of the valley of the Portillo descent, hillocks & little ridges of porphyry are separated by a sloping level even plain of white washed pebbles. — These pebbles are consist of the same porphyries as the hills & are distributed all round above & below as from a centre.

the grand range consists entirely of granitic rocks (& some greystone lava) higher up in the valley the pebbles consist entirely of such rocks. Passing the band of porphyries we again meet at some distance on the sloping platform numerous large pebbles & blocks of the granites. — These must have been carried out in the (inexplicable) manner by which always happens this so generally travel, but subsequently the sea must have rested quietly round the hillocks & no posterior tumult of the water could have happened.

I should state, that the mouth of the valley is here broard & directly opens onto the boundless Pampas. (a) — the elevation is entirely above the low eastern escarpements. — The river in this valley has cut merely a small channel. —

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(a) Probably about 4000 ft above the sea. —

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1835. Plain of Uspallata 14

The plain of Uspallata, from Miers must be 6000 ft above sea. & is said to extend 60 leagues to the North in this direction nothing can be seen over its horizon. — is from 10 to 15 miles broard. —

In general appearance strictly resembles the basin of Chili & still more the nit: of soda plain (3300 ft) at Iquique in Peru. — From the Uspallata side, broard level valleys enter, which like those in North of Chili are scarcely scored by water channel on its margins there are those little hills, which I have called [Isld]. — At its southern extremity the R. mendoza traverses it, to this a stream from the plain is united. — A cliff of 400 or 500 ft thick of stratified shingle is there seen: more within the plain, we have seen the curious effect fact of inclined layers of alluvium (as commonly called) capped by horizontal strata. Hence this lower extremity of the plain, has been more deeply hollowed or originally deeper & then refilled. — The quantity of the horizontal mass which has been removed by the river is immense; in the SE corner, there are some small irregular terraces, which may perhaps mask the fall of river. — Has the general covering of the plain been lacustrine or marine; (a) analogy would point out the latter: The plain is known to be drained by at least two

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(a) exits, this could only happen in a lake by some subterranean movement. — Most travellers however believe in a lake. — This argument which convinces me it is marine, is drawn from the fact, that the plain of Uspallata slopes up at very small angle, continuously to & forms the terraces which border each side of the R. Mendoza. & which extend far within the Cordilleras to a height which quite precludes the idea of their being part of a lacustrine deposit. — (N.B. I have overlooked the fact, that subsequent to its being a marine basin it might have been converted into a fresh water lake. — This lake could not have been deep, because the external barrier is not too much broken at a small elevation above the plain. — So that only part of upper layers could have been lacustrine & essentially it may certainly be considered as marine).

N B. As the plain rises considerably with gradual slope to the North, the lake must have been very deep to have covered the whole. —

The proof that Western line was elevated, beneath the sea