>  > Natural carbonic acid (extremely dilute, in the form of rain) does a
>wonderful job of preferentially dissolving nonbiogenic calcium carbonate
>cement without attacking biogenic carbonate (fossils), doesn't it? I have
>observed this occurring in a block of Wangaloa (early Paleocene) shellbed I
>had lying outside my Geology Dept for a few years. The shells, already
>partially exposed in this beach-boulder, have slowly become better-defined.
>But it is a slow process. There is a certain temperature range where it is
>most effective, but it will still proceed at any above-freezing temperature.
>
>Interesting. What would this temperature range be?

I'll have to check; it's not a technique I have tried.

>But certainly, in porous
>shell beds, the weathering zone will be deepened by long-term acid rain. The
>fresh part of the shell bed, on the average, will be farther underground
>than before. As collectors, how should we respond to this trend?

Get out and collect all the fossils...?