Dolomite (limestone) is comprised of calcium, magnesium, carbon, and oxygen. When exposed to high temperatures, 1900º to 2100º, the carbon is released in the form of carbon dioxide. The result is lime.
Lime was used as a component in many building materials, and at one time, several kilns, like these two, comprised a large industry in the late 19th and early 20th centuries.
Photo taken by J.R. Manning in July 2010