Clay is abundant in Texas and has a number of uses. Some goes to make portland cement, and some is baked or burned in a kiln to make brick, tile, sewer pipes, pottery, and other products. This kind of clay is obtained from Tertiary formations of the Gulf Coastal Plain, from Upper Cretaceous formations in central Texas, and from Pennsylvanian formations in north-central Texas. (You can locate Tertiary, Cretaceous, and Pennsylvanian rocks on the Texas geologic map, pp. 4-5.) A special kind of white burning clay that can be used to make chinaware is called _kaolin_ or _china clay_. It contains particles of the clay mineral _kaolinite_ as well as several other clay minerals. Deposits of china clay occur in southern Jeff Davis County and in Real County near Leakey, but none is being produced. Another kind of clay, _bentonite_, forms from weathered volcanic ash. Bentonite contains the clay mineral _montmorillonite_ and looks smooth and soap-like. Fresh samples of this clay are white, pale green, or pale blue, but dried-out or weathered samples are tan, brown, yellow, or reddish. When wet, bentonite absorbs water, swells, and then has a jelly-like appearance. Surface deposits of bentonite occur chiefly in Eocene Tertiary formations of the Gulf Coastal Plain, in Cretaceous formations of the Big Bend area of west Texas, and in Quaternary formations of the High Plains. [Illustration: Bentonite is used as a drilling-fluid additive in the rotary method of drilling for petroleum and gas.] Some bentonite is used to absorb unwanted coloring material in petroleum and in vegetable oils. It is then known as a _bleaching clay_. Bentonite bleaching clay is obtained from some of the Tertiary formations along the Texas Gulf Coastal Plain. It has been produced in Angelina, Fayette, Gonzales, Jasper, Walker, and other counties in this area. Another important use of bentonite, and of other clay, too, is as _drilling mud_. In the rotary method of drilling for oil and gas, mud is pumped down into the drilled hole. This mud carries the rock cuttings up to the surface, it cools the drilling tools, and it coats and seals the walls of the hole. Along the Gulf Coastal Plain, drilling clay is obtained from Tertiary formations. Common Opal. _See_ Opal. Copper Minerals (Chalcocite, Chalcopyrite, Malachite, Azurite) A number of minerals containing copper, such as _chalcocite_, _chalcopyrite_, _malachite_, and _azurite_, occur in small deposits in Texas. They are found chiefly in the Llano uplift area of central Texas, in the Van Horn area of Culberson and Hudspeth counties in west Texas, and in a group of counties in north-central Texas. Copper is an important element. Because it is an unusually good conductor of electricity (only silver, which costs much more, is a better one), it is used for many kinds of wires for switchboards, generators, motors, telephone and telegraph equipment, and light and power lines.