in the Pleistocene and Post- Pleistocene. 21 



least a partial cause of the southward deflections of rivers 

 which were established along the Atlantic margin in late Ter- 

 tiary time. Such a possibility should at least be added to the 

 group of multiple hypotheses which have been brought to 

 bear upon the problem. In the emergence of a sea bottom, the 

 more important control of the slopes is due to the slope of the 

 previous surface of sedimentation and the later local warpings 

 of the crust, but a component tending to make for deflections 

 toward the equator may be due to more broadly acting forces, 

 and at certain times and places it may rise to the role of an 

 important factor. 



Conclusions. 



To review in conclusion the factors which appear to enter 

 to a greater or less degree into Pleistocene and recent crust 

 movements, — First, we must recognize the climatic factors 

 connected with Pleistocene glaciation. These consist in the 

 abstraction of the vast quantities of ocean water which accom- 

 panied the development of the ice sheets ; the direct depression 

 under the load of ice ; a possible compensatory elevation in a 

 somewhat broad zone beyond ; a deferred, intermittent, and 

 possibly oscillatory readjustment upon the removal of the bur- 

 den of ice. 



But these factors related to glaciation must not be used 

 alone. There are other factors unrelated to climatic causes 

 which may be of equal importance. The second group of 

 causes includes the movements which tend to maintain the 

 equilibrium of large sections of the crust, affect the whole 

 ocean level and locally warp the lands, but do not involve 

 earth shrinkage. It is thought that they find their cause in a 

 lack of equilibrium of pressures upon that zone of the earth's 

 body which lies just below the level of isostatic compensation. 

 They are then the isostatic factors. The general rise of lands 

 which has marked the later geologic times may most probably 

 be placed in this category, due either to changes of external 

 load, or to changes in crustal density. This would include a 

 possible enlargement or deepening of portions of the ocean 

 basins. As continental rejuvenation is the chief effect, these 

 adjustments toward the maintenance of isostasy may be called 

 epeirogenic factors. 



The third group of causes are thought to be found in great 

 compressive movements in the lithosphere, which seem to be 

 due in turn to a shrinkage of the centrosphere. Locally they 

 may work against isostasy, more broadly they may start iso- 

 static movements. As their ultimate expression is in moun- 

 tain building along lines of weakness they may be classed as 

 orogenic factors. 



