The region once known as Pimeria Alta--now southern Arizona and northern Sonora--has for more than three centuries been a melting pot for the beliefs of native Tohono O'odham and immigrant Yaquis and those of colonizing Spaniards and Mexicans. One need look no further than the roadside crosses along desert highways or the diversity of local celebrations to sense the richness of this cultural commingling.

Folklorist Jim Griffith has lived in the Pimeria Alta for more than thirty years, visiting its holy places and attending its fiestas, and has uncovered a background of belief,...