at last they left behind the wastelands , and the desert yielded to milder scenery of green trees and yellow ground overgrown with thick grasses ; late in the morning , the belly of the dragon rumbled loud above them with his roar of greeting : answered momentarily with several voices from up ahead , and they came abruptly into view of an astonishing prospect : a vast moving herd of elephants , creeping slowly across the savannah as they tore at the shrubs and low-hanging branches in their path , and enduring with perfect docility the supervision of two dragons and some thirty men , who ambled along comfortably behind , only a few lengths short of the rear of the herd .