BEARINGS OF RADIOACTIVITY ON GEOLOGY 687 



to protect the residue in the deeper parts from liquefaction for 

 the time being and the continuation of the process extended the 

 protection to such residue as continued to persist. 



It is not necessary to offer evidence that ascent of liquid rock 

 took place in great quantities in the early geologic ages and has 

 been more or less active in all ages down to the present. One 

 of the extraordinary facts of the Archaean terranes is the extensive 

 lodgment of liquid rock in the crust, and even in later ages batho- 

 litic phenomena have attained surprising magnitudes. The 

 extrusion of molten rock at the surface was a very pronounced 

 phenomenon as late as the Tertiary and is still an active process. 

 As this extrusive action was widely distributed over the surface 

 at various altitudes and at various stages through great lapses of 

 time and yet was never really very massive when measured in 

 terms of earth- volumes at any one time or place, it is of critical 

 value here to note that the view built on the planetesimal hypothe- 

 sis appeals to a special set of conditions of liquefaction and extru- 

 sion which are peculiarly favorable for selective work in small 

 masses and unfavorable for general liquefaction. In this respect 

 the conditions it assigns stand somewhat in contrast with the 

 conditions usually assumed to be the natural inheritances from a 

 general molten condition. The inference that general liquefaction 

 would take place on any general rise of heat is natural enough in 

 a case in which the whole mass has been solidified from a previous 

 molten state, for such a mass might be presumed to return mass- 

 ively into its former state on a reversal of conditions; but the 

 heterogeneous condition of the mixed matter of the interior postu- 

 lated by the planetesimal view is not favorable to a simultaneous 

 fusion of the whole mass or any large continuous part of it unless 

 extrusion be restrained until a high temperature is attained. Such 

 restraint is here held to be dynamically inconsistent with the 

 mechanism and the stress conditions of the earth-body. In addi- 

 tion, therefore, to such a mixed state of material in the interior as 

 peculiarly to invite selective liquefaction as the temperature 

 slowly rose, the planetesimal view postulates a set of stress agencies 

 that worked co-operatively to effect extrusion as fast as liquid 

 matter accumulated in workable volume. 



