Mystery of the Black Diamonds — Phyllis A. Whitney

When Angie and Mark first look at the treasure map given them by the old prospector, they are unable to figure out its meaning or even decide whether or not it is a hoax. But when they persuade their parents to visit the ghost town of Blossom, one clue after another comes to light.

WHEN THE HOWLING BEGAN

Angie was jerked suddenly awake. The weird sounds seemed to be coming from the little pocket of rock outside the open door. They dropped to an unearthly whimpering, then rose in a shrill banshee cry as if some lost soul wailed out there in the night.

Angie's blood congealed with fright and her throat choked closed so that she couldn't utter a sound. Say lay tense beneath the covers, aware of moonlight in the doorway and that wild howling outside. Whether it was animal or human, she could not tell…