Geologic map that depicts the surface geology of Mason, Llano, and Gillespie Counties and parts of Menard, Kimble, San Saba, McCulloch, Edwards, Real, Kerr, Kendall, Blanco, Hays, Travis, Williamson, and Burnet Counties, Texas.

Precambrian sedimentary rocks and associated mafic and silicic igneous rocks in Central Texas underwent a single episode of regional metamorphism about 1,050 million years (m.y.) ago.

The magnetic iron-ore prospects of the Llano region of central Texas were investigated in a program which combined dip-needle and gravity-meter surveys by the Bureau of Economic Geology of The University of Texas and the U.S. Geological Survey with exploration by the U.S. Bureau of Mines.

This work was begun in 1964 and substantially completed in 1965; at that time conodont zones had been established in North America only for the Upper Devonian, by Clark and Becker (1960) for the Great Basin and Collinson, Scott, and Rexroad (1962) for the mid-continent.

We studied the Hickory Sandstone Member of the Riley Formation, Central Texas, to assess the diagenetic history of a cratonic sandstone, which, in spite of its great age, remains porous and permeable where it has not been buried more than 1 km.