FOUNDATIONS OF THE GREAT DEEP 47 



tion: how can the earth have the plasticity demonstrated by the 

 changes which, during the ages, have produced her rugged 

 relief and visible, complex structure? Flow has taken place in 

 the solid crust, but, according to clear deduction by geologists, 

 that more superficial flow has depended on vastly greater 

 plasticity of the subcrustal layer. Indeed, it appears that the 

 mountain chains of the lands are by-products of the horizontal 

 displacement of whole continents over the earth's body, and 

 that this horizontal motion of the crust is possible only because 

 the subcrustal layer is nearly or quite as weak as water. 

 Furthermore, as will be emphasized in the next chapter, the 

 weak substratum is world-circling, beneath ocean as well as 

 continent. Perhaps, too, this conclusion may yet give a basis 

 for solving a supreme puzzle, namely, the concentration of the 

 sial, the earth's lighter rock, in the continental sectors, leaving 

 the oceanic sectors without any continuous cover of sialic rock. 

 The stereogram of Figure 22 represents an attempt to por- 

 tray the earth-shells in a succession which is favored by some, 

 though not all, seismologists. In the section the sial of the 

 continents is indicated by thin, unshaded segments of the crys- 

 talline surface layer; the crystalline sima of the sea floor, by a 

 thin line in solid black. Just beneath that heterogeneous crust 

 is the vitreous sima of the substratum. The nature of deeper 

 shells and the state of the Iron Core, labelled as possibly a true 

 liquid, thought to be too mobile to transmit earthquake waves 

 of the Shake variety, are problems for the future. 



