Berthold Laufer - Geophagy (1930)
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This is the final post in the Berthold Laufer series. There's more but the remaining available books tend to be straightforwardly scholarly and the other interesting titles don't appear to have been digitized yet - Was Odoric of Pordenone Ever in Tibet? (1912), The Gold Treasure of the Emperor Chien Lung of China (1934), Oriental Theatricals (1923). You can view The Bird-Chariot in China and Europe (1906) at The Hathi Trust but I haven't based posts on that source because downloading requires that you be associated with a partner library.
When I was a kid there was a small embankment where the road by my father's office was cut through a hill. From time to time people would come by and dig handfuls of a white clay out of the embankment which it turns out they would eat. The story is that their diet was missing certain nutrients and this is how they filled that need. Whether that's true or whether that's just why everybody thought this practice existed wasn't clear. My father says he hasn't seen anybody do this in twenty or so years so maybe it's died out.
Laufer's Geophagy looks at this practice in a wide variety of cultures and times. (Though sadly he's a bit too hopeful noting "The days are gone when the discussion of a problem started with the Greeks and Romans whose importance in the history of civilization is not much greater than and in many respects inferior to that of the Asiatic nations.") Laufer notes that geophagy can result from various reasons - religious, culinary, severe hunger, experimentation and medical. He references several studies in dismissing the nutritional explanation though this is one instance where more recent information would be useful. Results of a quick Google search tend to support the idea that there's no nutritional benefit and perhaps unsurprisingly reveal that there's people even today who claim otherwise.
The bulk of the book, though, is basically a catalog of geophagy broken down by region. The reports tend to be the same whether it's Chinese famine-food, Siberian stone butter, Aztec cakes and so forth. Sometimes the earth material is eaten straight, sometimes mixed with other food. There's even a tiny dispute about whether the South American Otomac added crocodile fat to their clay or not. Why people think they're doing this, though, varies widely as noted in the introduction and that's where Geophagy is most interesting. It helps that Laufer approaches his sources with a critical mind, calling one writer "credulous" or noting where another is condescending (Laufer clearly has the idea of racism in mind but doesn't seem to label it as such).