(1KNKKAL C'ONSI DURATIONS 455 



General Considerations 



Geographic conditions in the eastern interior. Returning to 

 the system of which the coal-beds form a small part, it is to be 

 recalled that the formations represent an alternation of marine, 

 lacustrine, and marsh conditions. The cause of the alternation was 

 probably geographic, but it is not to be inferred that geographic 

 changes were more frequent at this time than during other periods. 

 Their record is conspicuous because the land was near sea-level, so 

 that extensive submergence and emergence resulted from slight 

 changes of relative level of land and sea. Equally frequent and 

 equally extensive movements would leave no such record of them- 

 selves, if the surfaces concerned were far above or far below sea- 

 level. It was oscillation just above and just below water-level (or 

 ba>e-level) which allowed the record to be so clearly preserved. 

 II"\v far the oscillations were due to warpings of the land, and how 

 far to changes in the level of the sea, cannot be determined; but when 

 \\e recall that the ocean-level must respond to every deformation 

 which affects its bottom, and to every stage of filling, it is strange 

 that its level is in a nearly perpetual state of change. 



I In general, it may be said that the movements of the crust 

 which have been of most importance, from the point of view of 

 continental or biological evolution, are not those which have 

 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 

 It-'t-l 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 common rate is probably much slower. 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 it has been estimated that four-fifths of the vegetable matter in 





