A hard, smooth, compact, [translucent] rock that is made up mostly of [cryptocrystalline] [quartz] is called [chert] or [flint]. It is white, black, or some shade of gray, brown, or pink, and its luster is waxy, slightly glassy, or dull. Chert is found in many creek and river [gravels] in Texas. It also occurs with [limestone], such as in the Lower [Cretaceous] Edwards Limestone of central Texas and in the [Ordovician] Ellenburger strata in the [Llano uplift] area. Chert also is found with the Ordovician rocks of the Marathon area of Brewster County.
Geologists do not agree on whether [chert] and [flint] are two names for one variety of rock, or whether each is a separate variety. Some, however, now give chert a geological meaning and flint an archaeological meaning. They use the word chert to describe geological [formations] or rock specimens. They give the name flint to the same rock when it has been used by Indians in making arrowheads, scribers, scrapers, and spearheads.
Quartzite
[Quartzite] is either a [metamorphic rock] or a [sedimentary] rock. (The sedimentary kind of quartzite is described with [sand and sandstone] on [p. 86].) [Metamorphic] quartzite is made up mostly of [quartz]. It forms when heat and [fluids] below the earth’s surface cause the grains and cement of a quartz [sandstone] to recrystallize. When this happens, the grains interlock and are no longer held together by cement. Metamorphic quartzite, like [sedimentary quartzite], is a hard, firm rock that breaks through the quartz grains instead of between them.
Ancient [Precambrian] [metamorphic] [quartzite] occurs at the surface in the [Llano uplift] area of central Texas, in the Van Horn area of west Texas, and in the Franklin Mountains north of El Paso in extreme west Texas.
Rhyolite
[Rhyolite] is a fine-grained or glassy [igneous] rock that commonly is [extrusive] or [volcanic]. It has a pink, red, tan, white, gray, purple, or black color. This rock, like [granite], is made up chiefly of [feldspar] and a silica mineral, such as [quartz], but other minerals may be present. Both rhyolite and granite form from the same kind of molten rock material. Nevertheless, even though their compositions are the same, these two rocks do not look alike. Their textures differ because granite forms slowly and rhyolite forms quickly.
Much of the Texas [rhyolite] formed from hot, molten [lava]. This lava flowed out onto the surface either through [volcanic] cones or cracks in the ground. Some of the lava cooled and hardened too quickly for mineral grains to develop. This rapidly cooled lava formed a rhyolite rock that is made up, at least partly, of glass. In many of the rhyolites, [crystalline] mineral grains were able to form, but these grains are extremely small, and you may not be able to distinguish them even with a magnifying glass. Some rhyolite, because it hardened from moving, flowing lava, has streaks and bands of different colors and textures. This rhyolite has flow structure.
One variety of [rhyolite] has easily seen crystals and grains of minerals, such as [feldspar], [quartz], and [mica], scattered through a mass of the tiny [crystalline] grains (in much the same way that raisins are scattered through a cake). The easily seen crystals and grains are called phenocrysts, and the rock itself is called a rhyolite porphyry.
Many rhyolites and [rhyolite] porphyries occur in the [Tertiary] [igneous rocks] of the Trans-Pecos country of west Texas. Just a few of these localities include the Barrilla Mountains of Jeff Davis and Reeves counties, the Chisos Mountains of Brewster County, the Chinati Mountains of Presidio County, and the Davis Mountains of Jeff Davis County.