354 A. KEITH buTLIXES OF ^\PPALACHIAX STRUCTURE 



minor episodes with a tremendons maximum at the end, as was actually 

 the case. 



In tlie elemeiit of continuity, therefore, the forces called on and the 

 visible effects diifer greatly. The continuity should be ajDparent not only 

 in time but in space. In regard to space, however, the evidences of 

 diastrophism are far from being uniform or world-wide at any one time. 

 The Tertiary revolution approached that condition, but the preceding 

 ones of the late Paleozoic were distinctly limited and variable. It can 

 not be seriously questioned that the contraction of the earth was greater 

 in the immense period of Paleozoic time than it was in the far shorter 

 Mesozoic, and the same is true for any other continuing cause. Yet the 

 effects attributable to pre-Mesozoic deformation are less widespread 

 th^n are those of the Tertiary, although we should expect the opposite. 

 Practically all of the theories depend on some continuing factor, like 

 contraction or difference of gravity. They require, therefore, both in 

 time and space, a simple, steady, storing up of strain. This is a con- 

 tinuous process and its release should be rhythmic — a condition which is 

 distinctly opposed by the large facts of mountain-building. This situ- 

 ation indicates, therefore, that there is another factor in the equation 

 which need not be of gradual or steady growth and which operates from 

 time to time with an excess of vigor. What the nature of this cause may 

 be will be discussed on later pages. 



COXTIXEXTAL CREEP 



In recent years several theories of mountain-building have been put 

 forward which may be classed as theories of continental creep. They are 

 characterized by the enormous amount of sliding or creeping which they 

 attribute to the continental masses. The first full expression of this sort, 

 although not the earliest mention of it, was made by F. B. Taylor in 

 1910. He figured the crust of the north half of the earth as having 

 moved southward away from the North Pole for hundreds and even 

 thousands of miles, and stated that this theory was an expansion and 

 modification of the ideas of Suess. 



In Suess's theory of 1884-1885. the mountain ranges of Asia, for in- 

 stance, were considered to have developed around the circumference of a 

 "vortex" or central area in southern Siberia, the oldest ranges being 

 nearest the vortex and the youngest at the outside. Thus the crust was 

 supposed to have moved in general southward in successive sections and 

 at different times. The foundations of his idea were the southward over- 

 thrusting visible and the trend lines or arcs of the Asiatic mountains. 

 No explicit statements were made by him about the cause of the pressure 



