This bulletin contains the principal data available as to the succession of rocks in Arizona and the general structural features in most portions of the State. It presents many new facts obtained in a reconnaissance of various districts during 1919 to 1922 to obtain data for parts of the Arizona Bureau of Mines geologic map of the State, and much material compiled from various published reports. No attempt has been made to utilize old material which, having been replaced by better data, is now 011lv of historic value. Details from published reports are omitted or gen- eralized as far as possible, as persons interested in more extended descriptions of stratigraphy or structure can consult the original sources of information indicated by th'e footnote references. In districts where the published data are meagre, or where they are here published for the first time, more details are given. Some descriptions of local geology are incomplete because of lack of knowledge. The bulletin consists of two parts: The first, a resume of the characteristics of the various formations, with all available new light on their classification; and the second, brief descriptions of many districts, mountains, plateaus, and other features, arranged under counties.