300 FKIDIJf)!- NA.NSKN. M.-N. Kl. 



au'l also lltosc made at llic < aincgie <^/copliysical Lahoralor\- iii'licate that 

 llic proMcm is a couiplicaU-fl one [cf. I'ailey Willis, 1920J. As the interior 

 fricli(-)n of solid rock, and ilms its absolute strength or rigidit}', is increased 

 by pressure to a certain limit, where the rock is potentially crushed, the 

 "zoiu' of flowagc", wlitTc llic rock may be considered to be plastic, lies 

 much deeper than was generally estimated. 



W'v ma}-, however, assume that this zone begins a great deal higher 

 than the zone of compensation. As high tem|)erature is essential to the 

 mobility of the rock, and as the temperature of the lithosphère increases 

 rapidly w ith depth, w f may assutne that at a depth of 50 to 60 kiloinetres 

 the rock is in what may be called a plastic state, and that its plasticity 

 increases with the dejith. 



We shall not here try to discuss whether at a certain depth there is 

 a continuous substratum of molten magma or n(jt. The chief point for 

 our consideration is that at some depth under the rigid surface of the 

 lithosphère there is a zone of flowage, where the rt)ck material, in what- 

 ever state it may be, is plastic and mobile. That it must be so, and that 

 this plastic substratum behaves to a certain extent like a viscous fluid, 

 seems to be fully corroberatcd by our investigations of the strandfiat and 

 the crustal movements of Norway after the last glacial epoch. 



The coefficient of viscosity of this plastic substratum is probably 

 ver}' higli, but even if it be as high as estimated l)y Schweydar [1921], 

 /'. e. ten thousand times that of sealing-wax at normal room temperature, 

 we must keep in mind that the pressure is also extremely high, and the 

 substratum, therefore, is responsive to changes of pressure, and possesses 

 a certain degree of mobility, so that in the course of time, as is proved 

 by our observations, it gradually adapts itself to the conditions of equili- 

 brium. 



It is, however, obvious that this must re(iuire a very long time. 

 On the one hanrl l)ecause the rigidity of the crust will offer great resistance 

 to deformation, and it will give way only very slowly, probably by shearing. 

 On the other hand because, as was just mentioned, the internal friction 

 of the plastic substratum is so very great. 



The flow in the plastic substratum will, therefore, be extremely slow, 

 and besides it will meet with great resistance, because, for instance, the 

 crust, in the zone surrounding a depressed area, will have to be lifted in 

 order to make room for the displaced plastic matter. 



Let us try to imagine what will happen, if the crust be gradually 

 pressed down by an increasing ice-cap, provided that the conditions are 

 fairly h.ydrostatic at a certain depth under the earth's surface. 



Supposing the ice-cap begins to be formed in the central area of an 

 extensive region like Fenno-Scandia, the load of the ice-cap will press the 

 crust flown in this central area, and in a zone surrounding it the crust 

 will be pressed up and will there form a kind of concentric wave, as in- 



