These silver minerals have been mined at a number of places in Trans-Pecos Texas. The largest silver mine in Texas, the Presidio mine, is located near Shafter in south-central Presidio County. It contains argentite, cerargyrite, and native silver, along with galena and several other minerals. This mine is not open now, but in the years between 1885 and 1942, it produced a large amount of silver along with some lead and gold. There are several other lead-silver mines in this Shafter area, but none has produced as much as the Presidio mine. In this mine, the silver minerals occur mostly in large, flat deposits in Permian limestone and other sedimentary rocks. The minerals are believed to have been deposited there—probably during Tertiary time—by solutions that came from hot magma far below the rocks. As they moved in along the layers of limestone, the solutions replaced portions of this rock with minerals containing silver, lead, and other elements. Later, water seeped into these deposits and dissolved some of the minerals. This dissolved material was then re-deposited, and it formed most of the minerals we now find there. No silver is being mined in Texas at present, but it has, in the past, been produced from other Trans-Pecos mines. Galena that contains silver (called _argentiferous galena_) has been mined at the Bird mine at Altuda Mountain (about 14 miles east of Alpine) in northern Brewster County. It also has been obtained from mines in the Quitman Mountains and in the Eagle Mountains of Hudspeth County. Some cerargyrite has been mined at the Plata Verde mine near the Culberson-Hudspeth County line. Several mines in the Van Horn area of Culberson and Hudspeth counties have produced silver along with copper. An important silver mine in this area is the now idle and flooded Hazel mine. (This mine is described with copper minerals on p. 52.) Smoky Quartz. _See_ Quartz. Soapstone. _See_ Talc and Soapstone. Specular Hematite. _See_ Hematite. Sulfur Sulfur is one of Texas’ most valuable minerals. It consists of only a single element, sulfur. This mineral has a resinous luster and is transparent to translucent. Sulfur ordinarily is yellow, but impurities cause it to look greenish, brownish, reddish, or grayish. When you rub it across a streak plate, it leaves a white or a pale-yellow streak. Sulfur has a specific gravity of 2.04 to 2.09 and is soft enough to be scratched by a copper penny. It breaks with a conchoidal to uneven fracture. When it gets hot enough (478° Fahrenheit), sulfur will burn. For this reason, it often is called _brimstone_.