by Viking, $18.95. Mr. Snow covers what has been learned of Indian cultures from the Atlantic to the Pacific, from Alaska to northern Mexico, from the Ice Age to the seventeenth century. This is a lot of time and territory. and the text is necessarily a terse summary of major points. It is also a sound, well-organized introduction to an enormous and fascinating subject. Many fine photographs by Werner Forman, Maps, index, chronological tables, bibliography..