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1832 November Buenos Ayres

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The city of Buenos Ayres is built on a bank somewhere about 40 feet high. This bank or cliff extends on both sides of the town & is the boundary of the main formation of the country. — This bank only approaches the sides of the R Plata at the place where the city stands, in others it is separated by an extent of level land, which is evidently the deposit of the present river. Even within the memory of man the embouchure of the Rio chuelo, which runs through the low land SW of the city, has altered its course. — I should suppose the line of bank was the boundary of the river at some former period. — It is composed on an earthy compact undurated pale coloured clay not effervescing with acids, with small cavities; precisely resembling that found at Bahia Blanca. — This extends as far as I went & is weathered into undulations & small vallies. — The Tosca, as at B. Blanca contains horizontal layers of concretionary nodules of white undurated marl 836; it also contains irregular beds of an exceedingly fine, not very pure sandstone 835. containing much lime: Within it are f minute serpentine cavities lined with black: The beds are thin & broken & separate penetrated by the Tosca. — The occurrence of lime in these two last substances, must be owing. I should think to its chemical separation from the mass of Tosca. — In summer time, any water which is evaporated by the sun. leaves

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1832 November Buenos Ayres

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a thin crust of saltpetre. — This as well as the two following facts have been stated to me on good authority. — At the distance of some leagues. on the Rio chuelo there is an extensive bed of Gypsum. — And that on digging through the ground to a considerable depth, solid rock is met with ?? Generally water is met with on digging through the superficial soil or a little way into the Tosca. — Judging from such small data. I should think the formation here & at Bahia Blanca are identical distant 360 miles: Is it too much suppose that the whole extent of the Pampas are a deposit from the Rio Plata; at a period very remote & under circumstances widely different from what we now see them ?? — For instance the occurrence of oyster shells in beds on the banks of the Parana nearly 300 miles from the present fresh salt water. — (at Bajada good authority): —

I inquired in 1833 & could hear nothing about this rock

76 verso

[Rio cuss]. Quartz Veins R. N. p. 87¹

Quartz Rock aqueous & igneous & [veins]

Refer to Macculloch for passages & names of Rocks

¹ Red notebook, p. 87e.

I inquired in 1833 & could hear nothing about this rock] added pencil in margin.

Refer to Macculloch for passages & names of Rocks] added pencil.