These two sets of terms are presented in the charts that follow. If you are unfamiliar with geologic terminology, refer to these charts as often as you need to while you read this book, as well as to the glossary on pages 114-118. Geologists divide rocks into three main groups, depending on their modes of origin. _Igneous rocks_ originate from molten material, cooling deep below the surface of the earth (intrusive igneous rocks) or flowing out and hardening at the surface (extrusive igneous rocks). _Sedimentary rocks_ are formed from broken or dissolved bits of other rock, washed by wind and water and deposited as layers of fragments or as chemical precipitates. They often contain fossil plants or animals. _Metamorphic rocks_ are pre-existing rocks (igneous or sedimentary) changed by heat, pressure, or chemical action. Examples of these three classes of rocks are given in the accompanying figure. Many varieties of all three classes occur in Colorado. Class Example Occurrence in Colorado Sedimentary Sandstone Plains, plateaus, flanks of mountain areas Shale Conglomerate Limestone Igneous Extrusive: Volcanic areas such as San Juan Basalt Mountains, Spanish Peaks Intrusive: Pikes Peak, Longs Peak, and most Granite central mountain areas Diorite Metamorphic Marble Mountain areas (from limestone) Quartzite (from sandstone) Gneiss (from granite or sandstone) Schist (from shale or basalt) Geologists arrange rocks in their chronologic sequence by studying the fossils and minerals which they contain. The age of some rocks can be determined with reasonable precision from ratios of radioactive minerals and their fission products. The relative age of others can be determined from their position, the fossils enclosed in them, and many minor details of their structure. The _stratigraphic column_ shown opposite may be thought of as a calendar by which geologic events in Colorado can be arranged in their proper order and related to events in the rest of the world. Mississippian and Pennsylvanian Periods are American divisions; elsewhere this time interval is known as the Carboniferous Period. Other time terms are in worldwide use. In the generalized geologic map of Colorado which accompanies Chapter II, rocks are identified by the era in which they were formed. A more detailed geologic map can be obtained from the U.S. Geologic Survey map distribution center in the Federal Building, Denver. [Illustration: Stratigraphic Column]