SECULAR MOVEMENTS 221 



segments of the earth, as though by vertical, rather than horizontal 

 forces. The great plateaus are examples of one phase of this action, 

 and perhaps the great "deeps" of the ocean bottom, and some of 

 tin' basins or troughs (Graben) of the continents, are examples of 

 the other. Most plateaus are made up of numerous blocks which 

 have been moved by different amounts. At the surface, these 

 blocks are separated by fault-planes, but below, some of the faults 

 pass into flexures. Plateau-forming movements are to be compared 



Fig. 220. Ranges of the Great Basin. Length of section, 120 miles. (Gilbert.) 



with continent-forming movements rather than with mountain-fold- 

 ings, differing from the former chiefly in magnitude. Plateaus 

 may be regarded as parts of a continental mass that have suffered 

 additional movement. Plateaus stand in some such relation to con- 

 tinents as one fault block of a plateau does to the whole plateau. 

 3. Continent-forming movements. These are widespread 

 movements affecting large masses of the body of the earth, if not its 

 whole outer portion. Two or more continents may be affected by 

 similar movements at the same time, and it is the view of many 

 geologists that all continents are affected simultaneously by move- 

 ments of a like kind, resulting in emergence or submergence, while 

 the ocean basins are affected by movements of the opposite phase. 

 These movements are regarded as reciprocal, and parts of a world- 

 wide adjustment. While well supported both by observation and 

 theory, this view is not universally accepted. Movements of this 

 class seem to have started early in the history of the earth, and to 

 have been renewed from time to time, rejuvenating the continents 

 and deepening the ocean basins. Under the view that the earth is 

 essentially solid throughout, these movements are regarded as 

 extending down to great depths, while mountain folding is regarded 

 as but the wrinkling of the earth's skin to fit its changed body. 



Downward movements are regarded as the primary ones, and 

 horizontal movements as a necessary result of them. The under- 

 lying cause of movement is believed to be shrinkage due to an in- 

 crease in the density of the earth, caused by gravity and by molec- 

 ular and sub-molecular attractions. Cooling is probably a lesser 

 cause of shrinkage. The master movement is thought to be the 



