Arizona may be divided into two principal parts on the basis of geological features. Most of the northern half, except in the extreme west, belongs to the Plateau province and is characterized by dominantly horizontal strata deformed by uplifts, downwarps, and high-angle faults. The remainder of the state belongs to the Basin-and-Range province, and is composed of linear ranges, mostly oriented with northwest-southeast trends, and separated by wide, deep valleys filled largely with Tertiary and later alluvium. Overthrusts and many compressional features are represented in the mountain structures.

To date no significant petroleum discoveries have been made in Arizona. The Basin-and-Range portion, furthermore, offers encouragement for prospecting in few places. Its valleys, except possibly in the extreme southwestern part of the region, appear to be filled with continental sediments; deep water wells have exposed nothing to justify optimism. In contrast, certain features of the Plateau province favor more exploration in that area.

Rocks similar in type and age to those forming parts of the plateau of northern Arizona have produced petroleum in adjoining areas of southern Utah and northwestern New Mexico. Studies of

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the distribution and thickness of these strata and of possible basins of accumulation within them, are discussed. Recent developments in exploration of this area are briefly summarized.

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