[Illustration: Jasper from Uvalde County, Texas. Dark areas are brownish red; light areas are a yellowish-tan.] Much agate has been found filling cavities in Cenozoic igneous rocks in Brewster, Presidio, and other counties in the Trans-Pecos country of west Texas. It has been found also in an area about 10 to 15 miles wide along the Rio Grande, mostly in southern Webb County and in Zapata and Starr counties. Trees and other plants have been replaced by agate. Many specimens of _agatized wood_ have been collected from Tertiary formations in Fayette, Gonzales, Lee, Washington, and other counties of the Gulf Coastal Plain. (The agatized wood, along with opalized wood, occurs within about 20 miles of the boundary between no. 2 and no. 3 on the geologic map, pp. 4-5.) A hard, compact, slightly translucent variety of cryptocrystalline quartz is called _jasper_. It commonly has a red, brown, or yellow color due to the presence of an iron oxide, such as hematite. Some jasper is made up of irregular bands of more than one of these colors. This variety of quartz often is polished to make attractive gem or ornamental stones. It has been collected at several localities in Texas, particularly from creek and river gravels. Starr and other nearby counties along the Rio Grande have furnished a number of good specimens. 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.