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Darwin Correspondence Project

To Alexander Burns Usborne1 [c. 1–5 September 1835]

The recent elevation of the land above the level of the ocean is shown by beds of shells lying on the surface of the land.— There is often a difficulty in distinguishing between such as have been left by the sea & those at some remote period brought by man.— Where the shells or fragments are in great numbers, packed in layers, either with or without earth, & forming a level mass, the former cause may be generally assumed as certain.— Such shells are brittle & decayed & their color partially lost.— If amongst them there are many very small ones; if the situation is remote from the sea, or inaccessible from the immediate Beach,—if no water is found in the vicinity, of course there is a probability they have not been carried there by any former residents.— If old Barnacles, minute Corallines, Serpulæ, or impressions of such are found adhering on the inner surface of shells (this happens chiefly in Spiral Univalves such as Welks &c) it is manifest they must have been lying dead (& therefore of no use as food) for some time at the bottom of the sea.— I have frequently found such shells on the coast of Chili at an height from 20 to 400 ft.— If you should meet with shells, thus circumstanced in any part of your survey, & especially to the North of Lima; I should be much obliged, if you would mark on Paper the name of the Place, & estimate carefully the vertical height.— It would be well always to state the amount of (& reasons for) conviction which you feel respecting their origin.— Where an opportunity occurs, especially if the elevation should be great, the observation would be of infinitely more service if an angle of elevation could be taken.— The oftener you can observe & record this class of facts, in different places, so much the better; for the evidence respecting the rise of land becomes cumulative.— I may mention that the layers or beds of shells, or such loosely scattered on the surface sometimes occur in steps, like ancient shingle beaches; or in small step-form terraces (

[DIAGRAM HERE] Sea).—

If such should happen it should be mentioned.— Also, the state of the shells, whether very old in appearance & broken, whether decomposing into a white powder, whether chiefly of one or various sorts.— Also the extent, breadth & depth of the bed. Again, it would be well, where the height of the locality is considerable to collect a few specimens of the different sorts.—

The most likely situations to find such shells, are on flat-topped points near the Coast or in an island.—

Small specimens of the prevailing rocks (not loose fragments) wrapped up in paper with attached label of the locality, from any part of the coast will possess considerable interest.— I may except Cobija Iquique, (but the small neighbouring ports are not excluded) Arica, Islay, & Callao, from all of which I have specimens.—

I cannot state too strongly the value of all shells, whether petrified or not, extracted from stratified cliffs of sand, clay, or stone.— Their localities being carefully marked, & nature of imbedding stone.—

The first class of observations are less troublesome & to me even of more interest & value.—

Footnotes

CD’s memorandum was sent by Usborne to Henrietta Litchfield on 15 September 1882 with a note of reminiscences of CD as a shipmate (DAR 207). CD wrote the memorandum at the time Usborne and Charles Forsyth were left behind to survey the coast of Peru (see Narrative 2: 483). No mention is made in South America of any specimens collected or of observations made by Usborne during the survey.

Summary

Memorandum of geological instructions to guide ABU during a voyage of [Constitucion] to survey the coast of Peru.

Letter details

Letter no.
DCP-LETT-285
From
Charles Robert Darwin
To
Alexander Burns Usborne
Sent from
unstated
Source of text
DAR 207: 14
Physical description
Amem 4pp

Please cite as

Darwin Correspondence Project, “Letter no. 285,” accessed on 21 August 2019, http://www.darwinproject.ac.uk/DCP-LETT-285

Also published in The Correspondence of Charles Darwin, vol. 1

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