Those who study the earth’s crust—its origin, history, rocks, minerals, fossils, and structure—are known as _geologists_. The geologists who are especially interested in a particular phase of _geology_, as this science is called, are given special names: those who study fossils are called _paleontologists_; those who study minerals are called _mineralogists_; those who study rocks are called _petrologists_. Time and Rock Units The earth’s crust is believed to be at least 3¼ billion years old. In order to deal with this vast stretch of time, geologists have divided the billions of years into various time units and have given each unit a name. The great divisions of geologic time, called _eras_, are Early Precambrian, Late Precambrian, Paleozoic, Mesozoic, and Cenozoic. These eras are divided into smaller units of time called _periods_, and the periods are divided into _epochs_. The _[xx time scale]_ shows the geologic time divisions. Earliest geologic time is shown at the bottom of the scale; most recent is shown at the top. By examining and studying the different rocks and rock layers, geologists try to discover in which unit of geologic time these rocks formed. Those rocks that formed during a _period_ of geologic time are called a _system_ of rocks; those that formed during an _epoch_ are called a _series_. For example, the Cambrian System of rocks formed during the Cambrian Period; the Cretaceous System of rocks formed during the Cretaceous Period; the Tertiary System of rocks formed during the Tertiary Period. We are now in the younger epoch (called Recent) of the Quaternary Period of the Cenozoic Era. The rocks that are forming now are the Recent Series of rocks. [Illustration: Geologic time scale] ERA PERIOD EPOCH CENOZOIC QUATERNARY (lasted 0-1 million years) Recent Pleistocene TERTIARY (lasted 62 million years) Pliocene Miocene Oligocene Eocene Paleocene —63 million years ago— MESOZOIC CRETACEOUS (lasted 72 million years) JURASSIC (lasted 46 million years) TRIASSIC (lasted 49 million years) —230 million years ago— PALEOZOIC PERMIAN (lasted 50 million years) PENNSYLVANIAN (lasted 30 million years) MISSISSIPPIAN (lasted 35 million years) DEVONIAN (lasted 60 million years) SILURIAN (lasted 20 million years) ORDOVICIAN (lasted 75 million years) CAMBRIAN (lasted 100? million years) —600? million years ago— LATE PRECAMBRIAN EARLY PRECAMBRIAN These time estimates are from the paper, Geologic Time Scale, by J. Lawrence Kulp, published in Science, Vol. 133, No. 3459, April 14, 1961. (The time divisions are not drawn to scale) [Illustration: Plate 10. GENERALIZED GEOLOGIC MAP OF TEXAS Modified from Geologic Map of Texas, 1933]