METHOD OF ATTAINING HORIZONTAL MOTION 375 



existed there may be rocks with the rigidit}^ which astronomers think is 

 required for the earth as a whole. 



The intrusion theory resembles the theory of suboceanic spread, for 

 each takes account of the same condition of greater density beneath the 

 oceans. Suboceanic spread^ however, must overcome the strength of the 

 solid crust before it can produce great results in lateral motion, and 

 there is no assurance that the motion will be cumulative and confined to 

 one belt. The intrusion theory also resembles that of isostasy in that 

 both appeal to gravity as the moving cause. So, in fact, do all theories 

 on this subject. Isostasy, however, starts with erosion of matter and its 

 deposition elsewhere as a cause of differences in gravity. Intrusion, 

 how^ever, has been independent of these factors and has often overcome 

 them. Finally, the mechanism suggested for the intrusion theory differs 

 from that of isostasy as fluid rock differs from the solid ; indeed, it offers 

 a means of making isostasy more effective. 



Ultimate Forces 



general statement 



A conclusion that the intrusion of batholiths is the immediate cause 

 of mountain-folding (or that any other cause is the immediate one) by 

 no means solves the final problem. A¥hatever is the immediate cause, 

 back of that is some initial force, and the question arises in this case, 

 What force lay behind the batholiths that compelled them to break their 

 way into the solid crust? Two initial forces and two only are available 

 for this work, namely, gravity and heat. 



HEAT 



The heat may be that which remained from the initial heat of the 

 earth after condensation from a nebula, or it may be that derived from 

 the slow accession of meteorites or from the direct heat of the sun. Prob- 

 ably all of these sources of heat are at work, but it is difficult to see why 

 an initial heat of the earth from condensation is not by far the greatest. 

 The sun's heat no doubt retards greatly the escape of the internal initial 

 heat, but no mechanism has yet been suggested for converting this heat 

 into a cause of deformation. It has, however, been urged that newly 

 deposited sediments cause a rise in the isogeotherms with attending ex- 

 pansion of the crust. Such a rise, however, is not due to the heat of the 

 sediments derived from exposure to the sun, but to the fact that the 

 sediments may act as a blanket to prevent the loss of the initial internal 

 heat. 



XXV— Bill. Gkol. Sor. Am., Vol. 3-1, 1022 



