For about ten years the average yearly production of gypsum in the United States has been approximately 2,500,000 tons, or about ten times that of the nearest foreign competitor (Canada). New York, Iowa, Michigan, and Ohio are the chief producers. In New York the rock gypsum (usually four to ten feet thick) lies between shale and limestone strata of Silurian age, and it is quarried from the central to the western part of the State. In Michigan the rock gypsum beds, commonly five to twenty feet thick, lie in Mississippian strata in the southern portion of the State. A great bed of exceptionally pure rock gypsum underlies about twenty-five square miles of Webster County, Iowa, in rocks of late Paleozoic Age. The Kansas gypsum deposits extend across the central part of the State in rocks of Permian Age. Rock gypsum is mainly used in making "plaster of Paris," as a retarder in cement, and as a fertilizer (so-called "land plaster"). GLOSSARY OF COMMON GEOLOGICAL TERMS Names of subkingdoms and important classes of fossil plants and animals, and mineral species, are not included; these being briefly and systematically discussed in chapters 17, 18, 19, and 20, respectively. By using the index the reader can quickly locate the page where any one of these names is discussed. Some definitions in this glossary are taken from U. S. Survey Bulletin No. 613. _Anticline._--A kind of folded structure in which strata have been bent upward or arched. _Archeozoic._--The earliest known era of geologic time. _Basalt._--A common lava of dark color and of great fluidity when molten. Basalt is less siliceous than granite and rhyolite, and contains much more iron, calcium, and magnesium. _Base-level._--The lowest level to which a stream can cut (erode) its channel. A whole region may be base-leveled by erosion. _Cambrian._--The first or earliest period of the Paleozoic era of geologic time. _Cenozoic._--The present era of geologic time. It began at least several million years ago. _Chalk._--A soft, fine-grained, white limestone consisting mainly of tiny shells. _Conglomerate._--A sedimentary rock consisting of consolidated or cemented gravel. Often sandy. _Cretaceous._--The last period of the Mesozoic era of geologic time. _Crystal._--A regular polyhedral form, possessing a definite internal molecular structure, which is assumed by a substance in passing from the state of a liquid or gas to that of a solid. Nearly every mineral, under proper conditions, will crystallize. _Crystalline Rock._--A rock composed of closely fitting mineral crystals that have formed in the rock substance, as contrasted with one made up of cemented grains of sand or other materials, or with a volcanic glass. _Crystallography._--The study of crystals. _Devonian._--The middle one of the seven periods of the Paleozoic era of geologic time.