ESTIMATE OF TIME. 377 



Appalachian Revolution. — This state of oscillation and incertitude 

 was cut short by the Appalachian revolution. At the end of the Coal 

 period, the sediments which had been so long accumulating in the 

 Appalachian region, until their aggregate thickness had now reached 

 40,000 feet, at last yielded to the horizontal pressure produced by in- 

 terior contraction of the earth (p. 263), and were crumpled, and mashed, 

 and thickened up into the Appalachian chain. At the same time the 

 Western coal-swamps were upheaved sufficiently to become permanent 

 dry land. This revolution closed the Carboniferous age and the 

 Palaeozoic era. 



Estimate of Time. 



We have said (p. 276) that it is important that the mind become 

 familiarized with the idea of the immense time necessary to explain 

 geological phenomena. We therefore embrace this opportunity to 

 make a rough estimate of the Coal period. The estimate may be made 

 either by taking the whole amount of coal in a coal-field as the thing to 

 be measured, and the rate at which vigorous vegetation now makes or- 

 ganic matter as the measuring-rod ; or else by taking the whole amount 

 of sediments in a coal-basin as the thing to be measured, and the 7' ate 

 of accumulation of sediments by large rivers as the measuring-rod. 

 We will give both, though the latter is probably the more reliable : 



1. From Aggregate Amount of Coal. — A vigorous vegetation — as, 

 for example, an average field-crop or a thick forest — makes about 2,000 

 pounds of dried organic matter per annum per acre, or 200,000 pounds 

 or 100 tons per century.* But 100 tons of vegetable matter pressed 

 to the specific gravity of coal (1*4), and spread over an acre, would make 

 a layer less than two thirds of an inch in thickness. But, according 

 to Bischof, vegetable matter in changing to coal loses, on an average, 

 four fifths of its weight by the escape of C0 2 , CH 4 , and H 2 (p. 356), 

 only one fifth remaining. Therefore, vigorous vegetation at present 

 could make only about one eighth of an inch of coal, specific gravity 

 1*4, per century. To make a layer one foot thick would require nearly 

 10,000 years. But the aggregate thickness in some coal-basins is 100 

 feet, 150 feet, or even 250 feet (p. 350). This would require — the for- 

 mer near 1,000,000, the latter 2,400,000 years. It is probable, however, 

 that coal vegetation was more vigorous than the present vegetation. 

 Our measuring-rod may be too short ; we will try the other method : 



2. From Amount of Sediment. — We are indebted to Sir Charles 

 Lyell for the following estimate of the time necessary to accumulate 

 the Nova Scotia Coal-measures. This coal-field is selected because the 

 evidences of river-sediments are very clear throughout. The area of 



* Recent researches considerably increase these numbers. Nature, vol. xvi, p. 211, 

 1877. 



