640 GEOLOGY 



affected high land or deep sea bottom, but those which have con- 

 verted sea bottom into land, or land into sea bottom. Such 

 changes are most likely to have taken place where land was low, 

 or water shallow. From the point of view of geology, therefore, 

 the critical level of crustal oscillation is the level of the sea. 



Duration of the period. So uncertain is our knowledge of the 

 duration of geological time that all sorts of data which can be made 

 to throw light on the subject are of interest, even though they do 

 not lead to trustworthy numerical conclusions. Under favorable 

 conditions, a foot of peat may accumulate in ten years or even less; 

 but the usual rate is probably much slower. Peat bogs are now 

 in existence in which the depth of the accumulated organic matter 

 is 40 or 50 feet, but the length of time involved is not known. 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 1,000 years, and compressed until its specific gravity 

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

 seven 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 four-fifths of it disappears in this way. 

 If this is true, the seven-inch layer would be reduced to less than one 

 and one-half inches, and a layer one foot in thickness would require 

 between 8,000 and 9,000 years. The aggregate thickness of coal 

 is frequently as much as 100 feet, and sometimes as much as 'JoO 

 feet. At the above rate of accumulation, periods ranging fmni 

 nearly 1,000,000 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 Carbon- 

 iferous 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 arc thou- 

 sands of feet, even where the sediments were largely fine and their 

 accumulation therefore probably slow. It would hardly seem 

 unreasonable to conjecture that their deposition may have con- 

 sumed an amount of time equal to or or even greater than that 



