298 PAST CLIMATES AND CLIMAXES. 



of the surface, the other to reduce them. Deformation produces a crust upon 

 which gradation then becomes active. But gradation tends to destroy itself, 

 and it can be rejuvenated as a process only by a new period of deformation. 

 Such a correlation of deformation and gradation seems to have been a periodic 

 process and to have marked great stages in the earth's development. 



Nature of deformation. — Crustal movements are grouped by Chamberlin and 

 Salisbury (1906 : 1 : 526) into (1) minute and rapid, and (2) slow and massive. 

 In the present connection, earthquakes furnish the only important examples 

 of the former. Their primary relation to succession is probably too sHght to 

 require consideration, but the secondary effects may have a decisive local 

 action upon vegetation. The chief consequences of this sort result from the 

 destructive action of tidal-waves in seas or in lakes, and from flooding or 

 draining, due especially to elevation or subsidence. Small bare areas may also 

 result from the fall of rock or soil masses, and from slumps or land-slides, as 

 well as from craterlets and fissures which emit vapor and gases. We have 

 practically no knowledge of the present development of vegetation in bare 

 areas due to earthquakes directly or indirectly. It seems likewise clear that 

 earthquakes were negligible as a factor in changes of vegetation during geo- 

 logical times. 



The slow massive movements are distinguished as (1) continent-making 

 (epeirogenic) and (2) mountain-making (orogenic). Chamberlin and Salis- 

 bury also class these movements "as (1) vertical movements and (2) hori- 

 zontal movements; and dynamically, as (1) thrust movements and (2) 

 stretching movements. It is to be understood that these distinctions are Uttle 

 more than analytical conveniences, for continental movements are often at 

 the same time moimtain-making movements; vertical movements are usually 

 involved in horizontal movements, and stretching usually takes part in the 

 processes in which thrust predominates, and vice versa. But where one phase 

 greatly preponderates, it may conveniently give name to the whole." 



These authors also distinguish between nearly constant small movements 

 and great periodic movements. The former are thought to have affected 

 nearly every portion of the earth's surface at practically every stage of 

 its history. The slow rise and fall of the crust seems to have occurred dur- 

 ing the times of great movement as well as during those of relative quiet. 

 Such gentle movements have had a part in the formation of epicontinental 

 seas, as well as in their disappearance. They have likewise elevated land 

 areas, and initiated or increased their erosion. These and other consequences 

 result all the more strikingly from the great crustal movements, and it seems 

 clear that the two kinds of movement differ only in degree. The gentle 

 rises and falls, however, have the pecuUar interest that they are apparently 

 going on at the present time, and their relations to vegetation can be made a 

 matter of actual experiment. 



Great periodic movements of deformation. — These are of three kinds: (1) 

 continent-forming movements, (2) plateau-forming movements, and (3) moun- 

 tain-forming movements. The first of these, the continental movements, are 

 regarded as having occurred very early in geological history, and to have 

 preceded the earhest sediments known. Consequently, continents and ocean- 

 basins must have early assumed their general forms and relations. The later 

 changes of continents must have consisted chiefly in the formation of moun- 



