Dolomite is a half brother of limestone. It is a calcium magnesium carbonate in which part of the calcium of limestone has been replaced by magnesium. This replacement is believed to have taken place while the carbonate precipitate on the sea bottom was still soft. Much recrystalliza-

Poorly Preserved Fossils
Poorly preserved snail typical of fossils found in dolomite. This specimen is both mold and cast; the mold of the outer shell is apparent as radiating lines; the cast of the inside of the first whorls protrudes in the rear.

tion of the stone occurred, a process which damaged or even destroyed most of the small, delicate fossils.

Casts and molds are more common in dolomite than in limestone, so much so that it is rare in dolomite to find fossils that still show original hard parts. Dolomite is usually grayer than limestone and may show crystal-lined cavities that once contained fossils. A drop of dilute hydrochloric acid in limestone will produce violent bubbling and an audible fizzing; a drop on dolomite will produce only a bubble or two unless the acid is warmed or the rock is powdered.

Calymene, the trilobite so widely collected from Milwaukee, Chicago, and Grafton, Illinois, is found as an internal cast, or steinkern, in the dolomite. Steinkerns of snails, corals, and brachiopods are associated with the trilobites, as well as poorly preserved internal casts of crinoids and cystoids. This belt of Silurian dolomites is exposed from Iowa to New York. Other Paleozoic dolomites outcrop in the Rocky Mountains.

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