Dr. Frate said dirt eating is one of the few customs surviving among some Southern blacks that can be directly traced to ancestral origins in West Africa. Dirt-eating is common among some tribes in Nigeria today.

According to his research, Dr. Frate said it was not uncommon for slave owners to put masks over the mouths of slaves to keep them from eating dirt. The owners thought the practice was a cause of death and illness among slaves, when they were more likely dying from malnutrition.

Instead of eating dirt, some women use packaged raw corn starch or baking soda as a substitute. Dr. Frate says these materials have a similar paste-like texture to the fine hill clays that have traditionally been eaten.

But not everyone makes that switch. ''I don't hold with either baking soda or starch,'' said Mrs. Glass. ''Starch just don't take the place of dirt.''

It is difficult to say how prevalent dirt-eating is today. But in 1975, among 56 black women questioned by Dr. Frate as part of a larger study on nutrition in rural Holmes County, 32 of them said they ate dirt. The survey also showed that the ingestion of dirt tended to be more common in pregnancy.

While it is was not unusual to find small boys who ate dirt, the practice appears to be shunned by adult males. Of 33 men questioned in the households studied, none said they ate dirt.

Dirt-eating has also been practiced among poor, rural whites, who in the early part of this century were known as ''clay eaters.'' A Certain Kind of Dirt