[Shale] looks very much like some clays. It, like [clay], can be almost any color. If the shale contains animal or plant matter, it is black, gray, or blue. If it contains iron oxide (many minerals containing iron alter to this material), it is a shade of red, yellow, or brown. Shale is soft and can be easily scratched by a knife. It also is brittle and crumbles easily. This rock has a property that will help you to distinguish it from clay: the particles that make up the shale were deposited in layers, and the shale splits into flat, thin flakes along these layers, which clay will not do.
[Shale] is fairly abundant in Texas, especially in [Mississippian], [Pennsylvanian], and [Cretaceous] [formations]. For example, Pennsylvanian shales are found at the surface in north-central Texas, in the area around the [Llano uplift] of central Texas, and in the Marathon and Solitario uplifts of west Texas.
Many of [shale]’s uses are the same as those of [clay]. Some of it can be used to make brick, tile, and other products, and some is often used instead of clay in making portland cement. Cement plants at Dallas, El Paso, Fort Worth, and Waco are located at places where [Cretaceous] limestones, which also are used in cement making, and Cretaceous shales are found near each other at the surface.
Oil [shale], from which petroleum can be obtained by heating, has been found in central Texas. It occurs in [Mississippian] [formations] in Lampasas, McCulloch, and San Saba counties. Because oil is much less expensive to obtain from wells, it is not produced from these shales.
Silver Minerals ([Argentite], [Cerargyrite], [Native Silver])
Silver has many uses. Like [gold], it is a beautiful metal that long has been used for coins and ornaments. A large amount of silver goes to make articles such as spoons, forks, platters, and trays. The photographic industry uses silver—much of the film for cameras is coated with a silver halide. Doctors and dentists use silver, too. The mixture that a dentist uses to fill teeth contains silver along with several other metals. Doctors sometimes use silver wire to fasten broken bones, and silver compounds and solutions, such as silver nitrate, are used in some kinds of medical treatment.
Perhaps more people have heard of legendary, lost silver mines of Texas than of the actual and important silver deposits found in the Trans-Pecos country of west Texas. Some of the west Texas [silver minerals] include [argentite], [cerargyrite], and [native silver]. Although the argentite and native silver commonly found there are mixed with [galena], a lead mineral, or with [chalcocite], a copper mineral, they also occur separately.
The [element] silver is found alone as [native silver]. When pure, it is rather easy to recognize. It is metallic and has a silver-white color that may tarnish to gray, black, or yellowish brown. Native silver is heavy (it has a [specific gravity] of 10.5) and soft (a pocket knife scratches it easily). When you rub it across a [streak] plate, native silver, unless it is tarnished, leaves a shiny, silver-white streak. This metal is so ductile that it can be drawn into a wire. It is also malleable and flattens when hit with a hammer.
Silver occurs as crystals, which are poorly shaped [cubes] and [octahedrons], or as irregular masses. It may have a net-like appearance (called reticulate), or it may be shaped like little needles (described then as acicular). It occurs in wires (then called filiform) or as scales or plates.
