An intelligent appreciation of Yellowstone's marvels is tied here to explanations of the relevant geological processes and to man's growing understanding of ecology. Thus a lesson on the workings of geysers becomes a bridge to a discussion of how the heated water flowing into geyser basins differs from thermal pollution; the forests' lodgepole pines introduce the modern idea of using controlled fires as a wilderness management technique; the parks' elk, buffalo and wolf populations spark a reminder of man's late realization of nature's beauty and balance. Kirk's advice to travelers is backed up by an explanation of why bear feeding is no longer allowed. Otherwise there's little consideration given to the conflict between tourism and preservation. But here unassuming, uncomplicated prose combines a respect for nature's lessons with an enjoyment of its diversity. Photos are adequate, though in some cases an indication of scale would be helpful.