Permian (PUR-me-un)—from the Province of Perm in Russia Pennsylvanian (pen-sil-VAIN-yun)—from the State of Pennsylvania Mississippian (miss-i-SIP-i-un)—from the Upper Mississippi Valley Devonian (de-VO-ni-un)—from Devonshire, England Silurian (si-LOO-ri-un)—for the Silures, an ancient tribe of Britain Ordovician (or-doe-VISH-un)—for the Ordovices, an ancient tribe of Britain Cambrian (KAM-bri-un)—from the Latin word _Cambria_, meaning Wales The Carboniferous period in Europe includes the Mississippian and Pennsylvanian periods of North America. Although this classification is no longer used in the United States, the term Carboniferous will be found in many of the earlier geological publications and on many of the earlier geologic maps. The periods of the Mesozoic era and the source of their names are: Cretaceous (cre-TAY-shus)—from the Latin word _creta_, meaning chalky Jurassic (joo-RAS-ik)—from the Jura Mountains of Europe Triassic (try-ASS-ik)—from the Latin word _triad_, meaning three In Texas, the Cretaceous has two divisions, known as either Lower Cretaceous and Upper Cretaceous or as Comanche series and Gulf series, respectively. These designations are for rocks of nearly equivalent age, and both sets of terms have been used by geologists and in publications. In this handbook, both sets of terms are used interchangeably, that is, Lower Cretaceous and/or Comanche series and Upper Cretaceous and/or Gulf series. The Cenozoic periods derived their names from an old outdated system of classification which divided all of the earth’s rocks into four groups. The two divisions listed below are the only names of this system which are still in use: Quaternary (kwah-TUR-nuh-ri) Tertiary (TUR-shi-ri) While the units discussed above are the major divisions of geologic _time_, the geologist usually works with smaller units of _rocks_ called _formations_. A geologic formation is identified and established on the basis of definite physical and chemical characteristics of the rocks. Formations are usually given geographic names which are combined with the type of rock that makes up the bulk of the formation. For example, the Beaumont clay was named from clay deposits that are found in and around Beaumont, Texas. THE GEOLOGY OF TEXAS The geologic history of Texas, like the geologic history of the rest of the earth, is recorded primarily in marine sedimentary rocks. These rocks provide some knowledge of the early geography and the first inhabitants of what is now the State of Texas. Most of these rocks were formed from sediments deposited in shallow seas which covered parts of the State at various times in earth history. By studying these rocks and their relations to each other, geologists have established a geologic column for Texas. Physiography