Arizona is truly a land of scenic beauty and geologic marvels. The arid. climate and great relief combine to expose geology in a dramatic fashion equaled by few other places in North America. This guidebook is the fourth of a series being issued by the Arizona Bureau of Mines to acquaint tourists and residents of this state with its geology. It is hoped that the text will appeal to the amateur and layman as well as professional geologists and engineers. This is not a textbook, however, and the conceptual aspects of geologic phenomena are not treated in detail. The reader should consult introductory textbooks to learn such things as why volcanoes erupt and mountain ranges rise. What we have attempted to do is to call to the attention of the public specific examples of geology which occur along our highways. We hope this will make your trip a little more satisfying.