National Geographic : 1969 Aug
Geological sleuth, Dr. Smith studies a spher ulite, a stone with a visible growth structure formed in obsidian. He saw thousands along Route 70, west of Guadalajara (map, opposite). Since the giant balls do not occur in obsidian and lack such growth patterns, he reasoned that they are not overgrown spherulites. In 1930, mining engineer Ernest Gordon, right, had found a single giant at the Piedra Bola mine, whose Spanish name means stone ball. After locating others nearby, he notified Dr. Stirling, who had seen similar spheres in the vicinity of Palmar Sur, Costa Rica. Country of the colossi: Dr. Smith's party had to travel by mule, horse, and burro to reach the richest site (page 298).