THE AGE OF THE WORLD. 573 



and been buried. The pressure and heat to which this organic matter was sub- 

 jected after such burial, eliminated the greater portion of the volatile gases and 

 converted it into coal. The time necessary for the accumulation of all the or- 

 ganic matter contained in all these strata, and in the subsidence and elevation, 

 and for the deposition of the sedimentary materials composing the rocky strata 

 interposed between these coal-seams, can not be estimated — even approximate!}-. 

 But we are sure it must have far exceeded all conceptions of duration that we can 

 form from our knowledge of human history. We have no data on which to 

 base any estimate of the rate of the accumulation of sedimentary matter, except 

 the present rate of sedimentary deposition at the mouths of rivers. The present 

 average amount of sediment carried down by the Mississippi amounts to enough 

 to cover one mile square to a depth of 268 feet annually ; or about one cubic 

 mile in twenty years. At this rate it is estimated that it would have required 

 about 1,000,000 years to have carried down and deposited the amount of mate- 

 rials that originally composed the rocks of the coal-measures in the coal-fields 

 noticed above. 



The time occupied in the growth of the vegetation that formed the various 

 beds of coal may be more nearly approximated. If we assume the amount of 

 vegetation growing on one acre at 2,000 pounds, this would give in 100 years 

 lao tons. This compressed into coal and spread over an acre of ground would 

 give a thickness of one-eighth of an inch of coal in a century; or 10,000 years 

 would be consumed in producing one foot of coal. This would require, to pro- 

 duce 150 feet, as found in some regions, no less than 1,500,000 years. But 

 probably the growth of coal- plants, with the very high temperature and heavy 

 atmosphere, laden as it was with carbonic acid that prevailed during the coal 

 age, would produce a much larger growth of vegetation than we have estimated, 

 and the time would be accordingly diminished. But making due allowance for 

 this, there is no question but the time required for the growth of all this vegetable 

 matter, the frequent subsidences and elevations, and the accumulation of ihe 

 sedimentary matter composing the rocks of this formation must have required 

 many hundreds of thousands if not millions of years. 



Another remarkable illustration of alternate elevation and depression, or 

 at least of alternate land and water surface is found in the Amethyst Mountain, in 

 Yellowstone Park. This mountain is made up of alternate layers of sand-stone 

 and conglomerates, and the remains of gigantic forest trees. The stumps Ei.d 

 trunks of these trees are found imbedded in the rock material of the mountain 

 at various altitudes; the stumps five to six feet in diameter, and the trunks sixty- 

 feet long. It would seem that a forest of these gigantic trees would flourish for 

 a time — perhaps for centuries — and then be overwhelmed with a flood, bringing 

 down sand, gravel, and broken stones, completely covering the site of the 

 forest. This would become the soil upon which another forest would spring up 

 and grow, only to be buried up as its predecessor was. And this process of 

 alternate growth and burial has gone on till from one to two score of these 

 gigantic forests have flourished and passed away. Since the last one passed 



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