Some idea of the tremendous length of the geologic ages may be gained by a consideration of the time which must reasonably be allowed for the accumulation of so many coal beds and their containing strata. It has been estimated that a luxuriant growth of vegetation would produce 100 tons of dried organic matter per hundred years. Compressed to the specific gravity of coal (1.4) this would form a layer less than two-thirds of an inch deep on an acre. During the chemical alteration of vegetable matter to coal about four-fifths of the organic matter disappears in the form of gases. On this basis, then, it would take about 10,000 years to accumulate the vegetable matter represented in a coal bed one foot thick. When we realize that the total thickness of the coal beds of the Pennsylvanian system of strata in the great mining regions is commonly from 100 to 250 feet, we conclude that the time they represent is from 1,000,000 to 2,500,000 years. It seems most reasonable that the time necessary for the deposition of the containing strata must have been at least as long. It is, therefore, a fair conclusion that the Pennsylvanian period lasted from 2,000,000 to 5,000,000 years. That the climate of the great Coal Age was warm (not tropical), very moist, and uniform, is borne out by such facts as the following, according to D. White: The succulent nature of the plants with their spongy leaves indicates prolific growth in moist, mild climate; lack of yearly rings of growth points to lack of distinct seasons; as in the case of many existing plants the aerial roots signify a warm, moist climate; plants of to-day nearest like the coal plants thrive best in warm, moist regions; vegetable matter at present accumulates best in temperate rather than tropical climates, because there decay is not so rapid; and the remarkable uniformity of climate over the earth is clearly indicated by finding fossil plants of almost or exactly identical types in rocks of Pennsylvanian Age from the Polar regions to the Tropics. The more remarkable plants of the great Coal Age time are described in the chapter on the evolution of plants (Plate 15).