It is determined as limestone with certainty by wetting with dilute cold acid; then it “bubbles” or effervesces, and eventually dissolves entirely. Ordinary or regular limestone contains the mineral calcite, but the magnesian variety of limestone, dolomite, contains the mineral dolomite, which does not effervesce freely in lump size in dilute acid, but which does effervesce when _powdered_ or when treated with hot acid or concentrated acid. The preferred acid to use is muriatic (hydrochloric, the “not-cut” soldering acid) diluted one part of acid to one part of water. Caution! This acid mixture should be stored in a glass or porcelain container away from children or animals! Acid strong enough to dissolve rock will ruin clothes, destroy flesh, and is poisonous! Dilute sulphuric (storage battery) acid will also give the effervescence test, and the acid of very strong vinegar will react with limestone slowly. In making the test it should be recognized that the limestone which acts as a cement in sandstone, or limestone impurities in shale will also effervesce, but those minor parts of the rock will dissolve and leave the residues of sandstone or shale, which are insoluble. [Illustration: A coarse-grained limestone effervescing in dilute muriatic acid. (This photograph and other close-up views taken by J. F. Barham and Allen Barnes, University photographers.)] [Illustration: Solid dolomite does not effervesce in dilute acid. Note the white rock powder scrapings adjacent.] [Illustration: Dolomite powder does effervesce in dilute muriatic acid. Not all dolomite is this fine in grain.] Some limestones are chemical deposits but many are consolidated accumulations of fossil shells and shell fragments—organic limestone. For example, a widespread limestone, the so-called Burlington limestone, extending across central Missouri, contains many crinoid stem fragments and plates, attesting to the abundance of crinoids living in the sea at the time this limestone was laid down. Crinoids are sea animals which, because of their branching structure and superficial resemblance to plants, have been nicknamed “sea lilies.” Except for calcareous cave and spring deposits, almost all limestone formations in Missouri contain a few fossils of animals which lived in the ocean, and therefore Missouri limestones are considered marine in origin. They offer evidence for the very interesting land-sea changes which this state has undergone in the geologic past. [Illustration: Limestone composed almost entirely of crinoid (marine animal fossil) stem plates, from near Columbia.]