Texas is second among quicksilver-producing States because of the Terlingua region, in the Big Bend of the Rio Grande. This region contains Cretaceous strata, largely calcareous, which grade upward into Tertiary volcanics, locally without break. The strata are cut by numerous intrusions, largely alkalic, and are extensively folded and faulted.Many of the lodes are near the base of the impervious Del Rio clay, some at higher horizons, and a few in intrusive masses. The solutions penetrated only along open passageways, such as major fractures or joints in zones of tension, and, especially at horizons above the Del Rio, in fractures related to fault zones.The quicksilver is believed to have been brought in by alkali sulphide solutions. Most other constituents that may once have been present in the solutions had been separated earlier. Precipitation resulted largely from mingling with groundwater. The comparatively low temperature and pressure at the shallow levels where groundwater was present aided the precipitation. As the then-existing zone of groundwater circulation has since been partly exposed by erosion, the lodes are necessarily rather close to the present surface. The gangue minerals, particularly the calcite, are believed to have been derived mainly from the sedimentary rocks with some addition of material from deep sources. The relation of the widespread bitumen to genesis is puzzling here, as in many other quicksilver deposits. Weathering is of slight economic importance. Many minerals that resemble supergene products, including the quicksilver chlorides, are thought to have resulted here from original deposition within the zone of vadose water.