The producing gas fields in the Interior Highlands are in the Arkansas Valley of central eastern Oklahoma and central western Arkansas. The production is chiefly from sandstones in the Atoka and Hartshorne formations of Pennsylvanian age. Most of the fields, which are remarkable for their long life, have been producing gas for at least 10 years, and some have produced for about 30 years. The area is exceptional in that most of the gas produced is entirely "dry," and the carbon ratios of the coals are so high that a priori no gas would be expected. The producing folds generally are simple anticlines, and ordinarily the less conspicuous domes are more productive than the steeply folded structures. In most anticlines the better production is obtained some distance down the f anks, and there are some minor synclines which produce a little gas. The initial production of the average commercial well ranges from 750,000 to 45 million cubic feet of gas per day.