[PRECAMBRIAN] ROCKS
The [Precambrian] rocks of Texas are composed of igneous and metamorphic rocks and some sedimentary rocks. Most of the Precambrian outcrops are in the Llano uplift and El Paso and Van Horn regions.
Alterations produced by vast amounts of time, heat, and pressure have obliterated any trace of fossils that may have been present in these rocks. With the exception of some questionable primitive plants collected in the Van Horn region, no [Precambrian] fossils have been reported from Texas.
[PALEOZOIC] ROCKS
Rocks of [Paleozoic] age are widespread in Texas, and rocks of each [period] are well exposed. Outcrops are found in the Llano uplift, North-Central Plains, and Trans-Pecos region. The most extensive exposures are of [Pennsylvanian] and [Permian] age, and the former are highly [fossiliferous] in parts of the North-Central Plains.
[Cambrian]
Rocks of late [Cambrian] age are exposed in the Llano, Marathon, and Solitario uplifts, and the Franklin Mountains near El Paso. These are sedimentary rocks consisting of conglomerates, sandstones, shales, limestones, and some dolomites.
Some of these formations are relatively [fossiliferous], but the specimens are commonly fragmental and very poorly preserved. Fossils that are apt to be found in the [Cambrian] rocks of the Llano uplift include brachiopods, gastropods, trilobites, and small rounded objects believed to have been formed by algae (primitive one-celled plants). In other parts of the State, Cambrian rocks are sparsely fossiliferous and the fossils consist primarily of fragmental brachiopods, trilobites, and algae.
[Ordovician]
[Ordovician] outcrops are present in the Llano uplift of central Texas and in the Marathon, Solitario, El Paso, and Van Horn regions of Trans-Pecos Texas. These are sedimentary rocks and consist largely of sandstones, cherts, limestones, and dolomites.