The generally accepted theory of origin of petroleum makes thick series of sediments which accumulate on the floors of land-locked seas the most likely environment for the generation of petroleum. Neither the permanent land nor the deep oceans serve so well. Seas of this character have occupied much of the area of the continental shelves in the past, fringing the crystalline shields of the continents and lying between these and the ocean basins proper. But certain parts of the earth's surface have long been particularly favorable for the existence of these "seas in the midst of the land." The present "mediterraneans" or great regions of land-locked seas on earth, lying in the depressed mobile segments between the continents, mark sites where similar seas have endured thro ghout much of earth history and should, if our theory of the origin of petroleum is correct, constitute first-rate territory in which to search for petroleum.

The principal regions of land-locked seas occupying intercontinental troughs or depressions in the earth's crust are the following.

1. The environs of the Mediterranean, Red, Black, and Caspian seas and the Persian Gulf, occupying the depressed segment between the continents of Africa, Europe, and Asia. In this region the magnificent petroleum resources of the Near and Middle East are now being developed.

2. The environs of the Gulf of Mexico and the Caribbean Sea, lying in the land-locked basin between the continents of North and South America. This region contains the most important petroleum resources so far developed in the Western Hemisphere; Trinidad, Venezuela, Colombia, Mexico, and the Gulf Coast area in the United States.

3. The environs of the shallow island-studded seas which lie between the continents of Asia and Australia in the Far East. Important petroleum resources have already been developed on the islands of Borneo, Sumatra, Java, and New Guinea and much promising territory remains to be explored in this region.

4. The environs of the land-locked Arctic Sea, lying in the north-polar depression between the continents of North America, Europe, and Asia. This region is almost wholly unexplored, but it is characterized throughout by conspicuous surface evidences of petroleum.