The clay is baked and often mixed with vinegar and salt. In northern cities, some women who crave clay have their favorite types sent by Southern relatives, while others consume Argo starch, which has similar properties, as a substitute.

Some poor whites in the South also eat clay despite strong social pressures not to. Although American physicians tend instinctively to condemn the practice, ''no current evidence indicates that normal consumption of clays in the American South is either beneficial or harmful,'' Dr. Vermeer said. Eating of excessive quantities can cause intestinal blockage.

Much more research is needed on the incidence and health effects of geophagy, experts in medicine, anthropology and animal physiology asserted recently at a symposium at the annual meeting in Philadelphia of the American Association for the Advancement of Science. One of the few certainties is that the chemical properties of soils eaten around the world differ greatly, making it unlikely that there is any single explanation of why people eat earth or how it affects them.

Although the few available studies usually attribute the practice to women and small children, the incidence among adult men may be underreported, according to Dr. Danford, who has found in studies of institutionalized, mentally handicapped individuals that many men consume dirt.

Elks and bears, raccoons and parrots, giraffes and zebras are among the many animals that have been spotted eating earth. In Kenya some elephants make perilous nocturnal climbs to hillside caves in search of their favorite dirt. Like people, animals tend to choose subsoils, usually claylike but sometimes sandy, according to Bernadette M. Marriott, a researcher in comparative medicine at the University of Puerto Rico. Sheep in New Zealand annually consume hundreds of pounds each, and agricultural scientists have debated whether to add soil to the rations of intensively produced livestock.

Some studies indicate that by eating dirt, animals are correcting dietary mineral imbalances. Dr. Marriott's research on monkeys in Puerto Rico and Nepal, for example, indicates that the animals are deriving a significant share of the iron they need from clay soils, which they consume from specific sites to which they return over and over. Possibly, she said, the monkeys also ingest bacteria in the soil that provide antibiotic or other pharmaceutical effects.

Rats eat clay to offset ingested poisons, according to Denis Mitchell of the University of Southern California. Rats, which are relatively indiscriminate eaters, often swallow toxic materials ranging from natural chemicals to pesticides.