These are 21 Texas county maps made in cooperation with the American Association of Petroleum Geologists and Society of Economic Paleontologists and Mineralogists, issued between 1929 and 1932 and revised in 1937. They show surface geology with columnar sections; no structure or oil field data are included. Scale 1 inch equals 4,000 feet, except the map for Brown County, which was revised in 1951 to scale 1 inch equals 1 mile.
Species of the genus Schwagerina occur in most of the outcropping limestones formed in the open, connected, epicontinental seas existing at the time of the initiation of the Permian fauna and flora.
They are highly organized foraminifera, of large size, and they occupy a limited thickness of rocks. They thus form valuable horizon-markers wherever they occur.