6o2 FRANCIS PARKER SHEPARD 



to show the almost continuous record of orogenic diastrophism since 

 the beginning of the Paleozoic. In compiling the table, pre-Cam- 

 brian diastrophic movements are not mentioned because of the great 

 uncertainty as to the exact time of their occurrence. The table 

 includes only a list of the most important disturbances which 

 occurred in the different subdivisions. Several assumptions were 

 made in compiKng the table. The time of diastrophism was sup- 

 posed not to be a point of time, but to have extended over a consid- 

 erable fraction of a period, let us say on the average about a third 

 of a period. This may sound arbitrary, since the length of the 

 periods vary, but probably no harm was done in making such an 

 assumption, because it is becoming well recognized that orogeny is 

 not as brief a process as it was formerly considered. The orogenic 

 disturbances in the table which are in italics were located definitely 

 in a fraction of a period or near the juncture of two periods by taking 

 into consideration the youngest disturbed beds and the first beds 

 deposited after the disturbance. Since by considering these defor- 

 mations alone, certain gaps were left, other orogenic disturbances 

 the time basis of which were more open to question were used in the 

 table. These last depend generally on the statements of reliable 

 geologists which do not include the complete stratigraphic evidence. 

 It can hardly be expected that the location of all these deformations 

 will meet the requirements of everyone. 



While the great warpings, which alternately submerge and 

 emerge much of the continents, are of course considered diastrophic 

 movements, their relation to the shortening of the earth's crust has 

 been questioned by many. Therefore the table will not make men- 

 tion of this type of deformation. 



Examination of the table leads one to wonder where the great 

 world-wide periods of quiet come in, unless they are all pre- 

 Ordovician. While the Hst probably contains inaccuracies, it seems 

 likely that there has been no time in the history of the earth, at 

 least since the Ordovician, when there has been an absence of impor- 

 tant orogenic movements. 



PERIODIC CONCENTRATION OF DIASTROPHISM 



Probably the question will have presented itself to some readers 

 as to the possibihty that diastrophism might have been continuously 



