THE PENNSYLVANIAN PERIOD. 583 



and in Nevada, 10,000 feet. In these regions, therefore, active depo- 

 sition was in progress, and so far as the sediments are clastic, they 

 were probably derived from the land area of western Nevada. For 

 further details as to thickness, see the preceding sections, pp. 557-562. 



• Duration of the Coal Measures Period. 



„ So uncertain is our knowledge of the duration of geological time 

 that all groups of data which can be made to throw light on the sub- 

 ject are of interest, even though they do not lead to trustworthy numeri- 

 cal conclusions. A vigorous growth of vegetation has been estimated 

 to yield annually about one ton of dried vegetable matter per acre, 

 or 640 tons per square mile. If this annual growth of vegetable matter 

 were all preserved for 1000 years, and compressed until its specific 

 gravity was 1.4 (about the average for coal) it would form a layer 

 about six inches thick. But a large part of the vegetable matter, even 

 in peat bogs, escapes as gas (C0 2 , CH 4 , etc.), in the making of coal. 

 It has been estimated that three to four fifths of it disappears in this 

 way. The six-inch layer would then be reduced to two tenths or to 

 one tenth of a foot, and a layer one foot in thickness would require 

 5000 to 10,000 years. The aggregate thickness of coal is frequently as 

 much as 100 feet, and sometimes as much as 250 feet. At the above 

 rate of accumulation, and with the above proportions of loss, periods 

 ranging from 500,000 to nearly 2,500,000 years would be needed for 

 the accumulation of such thicknesses of coal. 



It should be borne in mind, however, that much depends on the 

 rate of growth of Carboniferous vegetation, which is not known. On 

 the other hand, these figures refer to the coal only, not to the Coal 

 Measures. The greater part of the Coal Measures is made up of shale 

 and sandstone, and of these formations there are thousands of feet, 

 even where the sediments were largely fine and their accumulation 

 therefore probably slow. It would hardly seem unreasonable to con- 

 jecture that their deposition might have consumed an amount of time 

 equal to or even greater than that demanded by the coal. This would 

 double the above figures, making them something like 2,000,000 and 

 5,000,000 years respectively. These figures must be taken to mean 

 nothing more than that the best data now at hand indicate that the 

 Pennsylvanian period was a very long one. 



