One of the best ways to become acquainted with the geology of Texas is to study the _geologic map_ of the State (Pl. 10). A geologic map shows the distribution and age of surface rocks and may also indicate what kind of geologic structures are present. The types of rocks that crop out at the surface may be shown by means of symbols, colors, or patterns, and these are explained by a legend which accompanies the map. On Plate 10, colors are used to show the distribution and geologic age of the surface rocks of Texas. Reference to this map will give the collector some idea of the age of the fossils that might be found in a given area. Some special geologic maps may have the location of geologic structures and formation contacts indicated by means of symbols, such as dashed lines, arrows, and similar special markings. However, the map included in this publication does not show any of these special markings. 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.