REPORT OF THE DIRECTOR 1920-21 I45 



b Acceleration of rotation 



We have seen before that the principal factor in folding the Pre- 

 cambrian rocks undoubtedly was the considerable shrinkage of the 

 earth after full growth was attained, due to pressure condensation, 

 etc. in the interior. This shrinkage we have further seen may have 

 amounted to two or three hundred miles in the radius and 

 would have been especially intensive at the beginning or in 

 Precambrian time. It is to be presumed that such a considerable 

 shrinkage might cause an acceleration of the earth's rotation sufficiently 

 great to cause the crowding of the crust toward the equator that is 

 indicated by the east-west component and the close folding of 

 the Precambrian rocks in the northern and southern latitudes; but 

 the folding of the Precambrian beds is on the northern hemisphere, 

 as in eastern Canada and Asia, by a pressure coming from the south. 

 Nor could this acceleration account for the north-south trend lines 

 and the close folding of the equatorial belt, which then would be 

 the zone of tension. Chamberlin (op. cit., p. 59) shows that a 

 reduction of a radius by 200 miles (from 4160 to 3960 miles) would 

 reduce the day from 26 hours 29 minutes .08 seconds to 24 hours, 

 an accelerative effect that would be greater than the retardative 

 effect of the water tides and perhaps greater than all the tides 

 combined, but — it follows from his analysis — hardly large enough 

 to have served as controlling agency of the Precambrian folding, 

 even if the indicated direction of the pressure were not opposite to 

 that postulated by an acceleration of rotation. This factor can 

 hence also be excluded with a good margin of safety from those that 

 may have controlled the Precambrian trend lines. 



c Body tides 



There is finally to be considered the possibility of the influences 

 of body tides. Darwin, as stated before, has pointed out that if 

 bodily tides have influenced the arrangement of mountain ranges, 

 the direction at the equator must be north-south, toward north 

 they must strike northeast and toward south in southeast direction. 

 It will be seen that the equatorial belt of Precambrian folds would 

 fully agree with the postulated trend lines, but the northern hemi- 

 sphere gives in its Precambrian trend lines no clear evidence of 

 a dominating northeast, nor the southern hemisphere of a dominating 

 southeast direction; for while the right wings of the Eurasian system 

 (the Sinian moiety) and of the Laurentian system are characterized 

 by the northeast direction of folding, the opposite wing or moiety, 



