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. In the final stages of metamorphism great masses of granite and innumerable small pegmatite bodies intruded the older sedimentary and igneous rocks.

Approximately 800 sediment samples from stream beds and bay bottoms of water bodies that compose the Matagorda Bay system were collected and analyzed. Shell-sand-mud ratios and total organic carbon content were determined, and 20 trace elements were detected. In general, highest concentrations of trace elements coincide with deep areas of- water bodies, areas of mud accumulation, and areas with high total organic carbon content. Zirconium, calcium, strontium, and mercury do not follow the distribution patterns of most other elements.

The Mineral Resources of Texas map is designed to illustrate the historical and current (as of 1976) distribution of industrial mineral occurrences and production sites in Texas. Distributions of major rock types used in the mineral industry are also shown, including limestone, dolomite, ceramic clay, nonceramic clay, iron, sand and gravel, trap rock, granite, industrial sand, extrusive igneous rocks, talc, salt, and sulfur. The map also emphasizes the contribution that Texas makes to the National economy, reflecting Texas's position as one of the Nation's leading mineral producers.