Leonardo’s Mountain of Clams and the Diet of Worms Read Online Free PDF Page A

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    (All quotes, unless otherwise stated, come from the Leicester Codex as presented in the MacCurdy translation of Leonardo’s notebooks.)
    2. He observed that rivers deposit large, angular rocks near their sources in high mountains, and that transported blocks are progressively worn down in size, and rounded in shape, until sluggish rivers deposit gravel, and eventually fine clay, near their mouths.(I learned this rule as principle number one on day number one in my college course in beginning geology.)
    When a river flows out from among mountains it deposits a great quantity of large stones . . . And these stones still retain some part of their angles and sides; and as it proceeds on its course it carries with it the lesser stones with angles more worn away, and so the large stones becomesmaller; and farther on it deposits first coarse and then fine gravel . . . until at last the sand becomes so fine as to seem almost like water . . . and this is the white earth that is used for making jugs.
    3. The presence of fossils in several superposed layers proves their deposition at different and sequential times.
    4. The tracks and trails of marine organisms are often preserved on beddingplanes of strata: “How between the various layers of the stone are still to be found the tracks of the worms which crawled about upon them when it was not yet dry.”
    5. If both valves of a clam remain together in a fossil deposit, the animal must have been buried where it lived, for any extensive transport by currents after death must disarticulate the valves, which are not cemented together inlife, but only hinged by an organic ligament that quickly decays after death. (This principle of inferring transport by noting whether fossil clams retain both valves persists as a primary rule of thumb for everyday paleoecological analysis. I doubt that any pre-nineteenth-century geologist mentioned this observation in more than a casual manner, while Leonardo regarded the argument as central.This observation first inspired my undergraduate awe for Leonardo, for I had just learned the rule in class and had thought, “How clever; how modern.”)
    And we find the oysters together in very large families, among which some may be seen with their shells still joined together, which serves to indicate that they were left there by the sea and that they were still living.
    At another site,on the other hand, Leonardo inferred extensive transport after death:
    In such a locality there was a sea beach, where the shells were all cast up broken and divided and never in pairs as they are found in the sea when alive, with two valves which form a covering the one to the other.
    6. Leonardo often illustrates the so-called uniformitarian principle of using observations about current processesto infer past events. In a striking example, he notes how far a cockle can move in a day in order to understand the spatial distribution of shells in a layer of fossils:
    It does not swim, but makes a furrow in the sand, and supporting itself by means of the sides of this furrow will travel between three and four braccia in a day. [A braccio , or “arm,” measured about two feet.]
    7. No marinefossils have been found in regions or sediments not formerly covered by the sea.
    8. When we find fossil shells broken in pieces, and heaped one upon the other, we may infer transport by waves and currents before deposition:
    But how could one find, in the shell of a large snail, fragments and bits of many other sorts of shells of different kinds unless they have been thrown into it by the wavesof the sea as it lay dead upon the shore like the other light things which the sea casts up upon the land?
    9. The age of a fossil shell can often be inferred from growth rings that record astronomical cycles of months or years. (Sclerochronology, or the analysis of periodicities in growth, has only become a rigorous and important subject in paleobiology during the current generation.) We