The Bofecillos Mountains area of Trans-Pecos Texas contains a Tertiary volcanic vent and a varied sequence of lava flows, tuff, ash-flow tuff, and associated conglomerate, sandstone, and mudrock; after most of the volcanic activity had ceased, the area was block faulled and later dissected into a

Since production of fluorspar began in northern Coahuila, Mexico, in 1950-51, thousands of tons of high-grade ore have been mined from numerous deposits situated 5 to 75 miles south of the Rio Grande.

The rim rock of the Vieja Rim, the quartz pantellerite of Lord, is named the Bracks Rhyolite.

The Tascotal Mesa quadrangle is in southeastern Presidio and southwestern Brewster counties, Trans Pecos Texas. Bandera Mesa and Tascotal Mesa are reached most easily from Marfa, 50 miles north, by the Marfa-Lajitas road which traverses the quadrangle from north to south.