GEOLOGICAL IMPORTANCE OF SEDIMENTATION 455 



uplifts which would cause the sea bottom to just emerge above the 

 sea-level when coal-swamps would form, while subsidences would 

 carry it downward, maintaining depths of 100 feet or more during 

 which sediments were being deposited. 



Upward oscillations are, of course, not to be excluded, but in 

 regard to the major cause of land-surface reclamation the following 

 may be quoted from Geikie: 



It has been assumed that, besides depression, movements in an upward 

 direction were needful to bring the submerged surface once more up within the 

 Hmits of plant growth. But this would involve a prolonged and almost inconceiv- 

 able see-saw oscillation; and the assumption is really unnecessary if we suppose 

 that the downward movement, though prolonged, was not continuous, but was 

 marked by pauses, long enough for the silting-up of lagoons and the spread of coal 

 jungles.' 



LeConte emphasizes the same conclusion as to the cause of the 

 alternation of strata; it being due not to crustal oscillation, but to 

 the operation of two opposing forces, one depressing (subsidence), 

 the other upbuilding (river deposit), with varying success.^ 



Dana also states that, when 

 under verdure, the surface must have lain for a long period almost without motion ; 

 for only a very small change of level would have let in salt water to extinguish 

 the life of the forests and jungles, or have so raised the land as to dry up its lakes 

 and marshes. Hence the grand feature of the period was its prolonged eras of 

 quiet, with the land a little above the sea-limit. ^ 



It is inconceivable that uplifts should have terminated and the 

 land rested quietly almost indefinitely when it was brought exactly 

 to the sea-surface, and it has been shown in the earlier part of this 

 article that the sea and land always tend to be differentiated. 



It is seen then that, supposing a small subsidence of, for example, 

 100 feet to take place, the river building would on the landward 

 side keep pace with it. The coal-jungles would first be quietly 

 flooded with fresh-water lagoons, as over portions of the Ganges 

 delta. In these clays would be laid down, quietly burying and pro- 

 tecting the extinguished forest, and preserving within itself numerous 

 fossils of ferns and leaves. Following this, the shifting river dis- 

 tributaries would deposit sand, or possibly even gravel, the whole 



1 Text-Book of Geology, 4th ed. (1903), p. 1018. 



2 Elements of Geology, revised ed., p. 390. 



3 Manual of Geology (1895), p. 708. 



