>I personally believe that one of the more likely reasons for the
>remarkable robustness over the last 75 years of the terms that Eskola
>coined is that they are reasonably genetically neutral, i.e.,
>usefully descriptive, although compositionally derived.  Petrogenetic
>fads have come and gone through the twentieth century, but rock
>nomenclature (igneous or metamorphic) that avoids genetic
>implications and overly specific geographic references tends to
>persist, as Eric suggests.

I would extend this excellent paragraph from Bob Tracy further.  The facies
concept is robust also because it contains no implication of P,T or other
physical conditions.  It is simply an association of rocks or mineral
assemblages, from which we infer physical conditions.



Howard W. Day, Professor
Dept. of Geology
UC Davis
Davis, CA 95616

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