EXPLANATION CENOZOIC 1 Quaternary 2 Tertiary (Oligocene, Miocene, and Pliocene) 3 Tertiary (Eocene) 4 Volcanic (extrusive) igneous rocks MESOZOIC 5 Upper Cretaceous (Gulf series) 6 Lower Cretaceous (Comanche series) 7 Jurassic 8 Triassic PALEOZOIC 9 Permian 10 Mississippian and Pennsylvanian 11 Cambrian, Ordovician, Silurian, Devonian and undivided Paleozoic 12 Rocks (Precambrian) older than Paleozoic 13 Intrusive igneous rocks (Precambrian, Mesozoic or Cenozoic) These rocks are found either at the surface or directly beneath the soils and subsoils which cover most of Texas. Geologists also subdivide rocks into lesser units. One of these, called a _group_, is made up of two or more _formations_. A _formation_ comprises rocks or strata (layers of rock) that are recognized and mapped as a unit. Some formations consist of layers of one particular type of rock, such as limestone or shale. Formations are named after a nearby geographic locality, and in some formation names, the type of rock is included. For example, three of the Texas geologic formations are called Buda Limestone, Del Rio Clay, and Eagle Ford Shale. Geologic Map The _geologic map_ (pp. 4-5) shows the rocks that are found at the surface in Texas. Some of these are extremely old. Some, geologically speaking, are very young. WHAT ARE ROCKS AND MINERALS? Although _rocks_ and _minerals_ are often mentioned together, and to some people they have similar meanings, geologists make a distinction between the two words. In general, rocks are made up of minerals, and minerals are made up of chemical elements. Chemical Elements The _chemical elements_ include oxygen, silicon, calcium, sulfur, carbon, gold, silver, and many others. There are 90 naturally occurring elements. Each is made up of molecules that consist of only one kind of atom. Chemical elements may either be combined with each other or occur alone. They are the building blocks of our world for they make up all the gases, all the liquids, all the minerals, all the plant and animal life, and all the other physical matter. Some of the chemical elements that occur in the rocks and minerals mentioned in this book are listed below.