70 ILLINOIS ACADEMY OF SCIENCE. 



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 Archean terranes is the extensive 

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

 litic phenomena have attained surprising magnitudes. The extru- 

 sion 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 

 extrusion 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 asumed to be the natural inheritances from 

 a general molten condition. The inference that general liquefac- 

 tion 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 pre- 

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

 massively into its former state on a reversal of conditions ; but 

 the heterogeneous condition of the mixed matter of the interior 

 postulated by the planetesimal view is not favorable to a simul- 

 taneous 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 incon- 

 sistent with the mechanism and the stress conditions of the earth 

 body. In addition, therefore, to such a mixed state of material 

 in the interior as to peculiarly 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. 



In considering stress effects, it is necessary to scrupulously dis- 

 tinguish between hydrostatic stresses which operate equally on all 

 sides of a given unit and so only produce compressive and like 

 effects, and differential stresses which promote movement and 

 change of form. The effect of differential stresses on the solid 

 parts of the earth is primarily to produce strains ; the effect on 

 Hquid parts is primarily to produce flow and relocalization. And 

 so by reason of this difference of effect, a general differential 



