DEFORMATION AND GRADATION. 299 



tains and plateaus, the appearance and disappearance of epicontinental seas, 

 and the leveling due to gradation. Plateau-forming movements are as yet 

 Kttle understood, but they are regarded as much more massive than mountain- 

 forming ones, and hence as standing next in magnitude to continental move- 

 ments. Chamberlin and Salisbury (1906: 1: 544) suggest that "plateaus may 

 be regarded as smaller platforms superposed on the continental platforms." 

 They are due to the elevation of great blocks of the crust, often with various 

 areas tilted individually, as in the great plateau of western North America. 



The mountain-forming or orogenic movements were the most frequent and 

 the most universal of the great processes of deformation. From the extent 

 and height of the resulting mountain ranges, and from their frequent if not 

 regular location along the ocean margins, orogenic movements must have 

 played a leading part in the differentiation of past climates, particularly when 

 associated with a marked change in the ratio of land and water surfaces. 



The causes of great crustal movements lie beyond the scope of the present 

 treatise. It is significant, however, that the elevation of the land into conti- 

 nents, plateaus, and mountains seems to be a consequence of the sinking of 

 ocean basins. As a result, the extent of the land surface as well as its eleva- 

 tion was greatly changed from time to time, with profoimd effect upon climate 

 and vegetation. In other words, the original continental platforms provided 

 a vast area for the differentiation of climates and vegetation as a consequence 

 of the elevation and sinking of mountains and plateaus, the invasions and 

 retreats of epicontinental seas, and the gradational relations between land 

 and ocean. 



Sequences of deformation. — ^While changes of climate and hence of vegeta- 

 tion may occur without def ormation,the latter must have initiated great changes, 

 in addition to modifying the changes induced by other causes. The effect of 

 deformation as a primary cause of change may be direct or indirect. It may 

 act directly upon circulation, gradation, climate, or vegetation. The effect 

 of circulation may be exerted upon gradation or climate, and that of gradation 

 upon climate to some degree as well. Gradation has also a direct effect upon 

 vegetation, while the climate as determined by the sun or modified by the 

 processes indicated here exerts the basic control upon vegetation through its 

 control of the climax. The sequences of deformation may be brought out 

 clearly by means of the graphic outline shown in figiu-e 23. 



Circulation y 



e 

 Climate »- £ 



Gradation ''^^^ » . 



? t 



' o 



FiQ. 23. — Diagram of deformation sequences. 



From this point of view, we may distinguish a circulation-climate sequence, 

 a gradation sequence acting directly upon climate and then upon vegetation 

 and directly upon vegetation itself, and a climate sequence. There is also a 

 direct vegetation sequence, but its effects are merged with those of gradation 

 and climate. Chamberlin and Salisbury (1906 : 2 : 656) have chosen the Per- 



