Geological Observations on South America · Charles Darwin
LIMA.
1. Mytilus Magellanicus: same as that found at Valparaiso, and there stated to be probably distinct from the true M. Magellanicus of the east coast. 2. Venus costellata, Sowerby “Zoological Proceedings.” 3. Pecten purpuratus, Lam. 4. Chama, probably echinulata, Brod. 5. Calyptraea Byronensis, Gray. 6. Calyptraea radians (Trochus, Lam.) 7. Fissurella affinis, Gray. 8. Fissurella biradiata, Trembly. 9. Purpura chocolatta, Duclos. 10. Purpura Peruviana, Gray. 11. Purpura labiata, Gray. 12. Purpura buxea (Murex, Brod.). 13. Concholepas Peruviana. 14. Nassa, related to reticulata. 15. Triton rudis, Brod. 16. Trochus, not yet described, but well-known and very common. 17 and 18. Balanus, two species, both common on the coast. These upraised shells appear to be nearly in the same proportional numbers- -with the exception of the Crepidulae being more numerous—with those on the existing beach. The state of preservation of the different species differed much; but most of them were much corroded, brittle, and bleached: the upper and lower surfaces of the Concholepas had generally quite scaled off: some of the Trochi and Fissurellae still partially retain their colours. It is remarkable that these shells, taken all together, have fully as ancient an appearance, although the extremely arid climate appears highly favourable for their preservation, as those from 1,300 feet at Valparaiso, and certainly a more ancient appearance than those from five to six hundred feet from Valparaiso and Concepcion; at which places I have seen grass and other vegetables actually growing out of the shells. Many of the univalves here at San Lorenzo were filled with, and united together by, pure salt, probably left by the evaporation of the sea-spray, as the land slowly emerged. (The underlying sandstone contains true layers of salt; so that the salt may possibly have come from the beds in the higher parts of the island; but I think more probably from the sea-spray.
It is generally asserted that rain never falls on the coast of Peru; but this is not quite accurate; for, on several days, during our visit, the so-called Peruvian dew fell in sufficient quantity to make the streets muddy, and it would certainly have washed so deliquescent a substance as salt into the soil. I state this because M. d’Orbigny, in discussing an analogous subject, supposes that I had forgotten that it never rains on this whole line of coast. See Ulloa’s “Voyage” volume 2 English Translation page 67 for an account of the muddy streets of Lima, and on the continuance of the mists during the whole winter. Rain, also, falls at rare intervals even in the driest districts, as, for instance, during forty days, in 1726, at Chocope (7 degrees 46′); this rain entirely ruined (“Ulloa” etc. page 18) the mud houses of the inhabitants.) On the highest parts of the ledge, small fragments of the shells were mingled with, and evidently in process of reduction into, a yellowish-white, soft, calcareous powder, tasting strongly of salt, and in some places as fine as prepared medicinal chalk.
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