ES TIM A TES A ND CA USES OF CR US TA L SHOR TENING 6 3 



minutes, i. e., in about 10,500,000 years the period changed 8 

 hours and 45 minutes. For the entire 46,300,000 years which 

 have since elapsed the change in period was from 1 5 hours 30 

 minutes to 24 hours or a change of 9 hours 30 minutes, but a 

 little more than the change for the previous 10,500,000 years. 

 At the present time, changing rotation has ceased to be a cause 

 for mountain-making of any importance, for, according to Cay- 

 ley, 1 the acceleration of the moon's motion due to tidal friction 

 is less than 6 seconds per century. 



The chief effects in mountain-making of changing oblateness 

 and increased pressures resulting from change of rotation, as 

 noted by Peirce in reference to the former, would be concen- 

 trated in the equatorial regions. The mountains are more numer- 

 ous and higher at low latitudes than at high latitudes. The only 

 way that this can be attributed to decreasing rotation is to sup- 

 pose that the mountain-making localities were determined by 

 the changes due to these causes, and that subsequent deforma- 

 tions have continued along the old zones of weakness. 



However, decreasing oblateness and increasing pressures are 

 available to explain the great deformation of the older rocks, 

 and especially those of the Archean and Algonkian eras. 



The amount of contraction which can be attributed to loss of 

 heat is also a steadily decreasing quantity. However, in vulcan- 

 ism we find a cause for crustal corrugation perhaps as potent now 

 as at any time since the beginning of the Algonkian. Indeed, 

 as has been seen (p. 48), the greatest volcanic epoch of 

 which we have certain knowledge is late Tertiary time, and con- 

 temporaneous with this was the great Tertiary period of mountain- 

 making. 



It is clear that the explanation offered for crustal deforma- 

 tion is complex. The theory is a combined contractional and 

 transfer theory. Moreover, the contraction, instead of being 

 assigned to a single cause, secular cooling, is assigned to this and 

 to increased pressure and changing physical condition. Also 



1 On the secular acceleration of the mean movement of the moon, by Arthur 

 Cayley: Monthly Notices, Roy. Astr. Soc, Vol. XXII, 1862, pp. 171-230. 



