DEPTH OF ZONE OF FLO WAGE. 189 



suspended at the ends, has sagged 3.8 centimeters in the middle." If it had 

 been attempted to bend the slab at the outset to this extent, undoubtedly 

 it would have been ruptured. The change in form without rupture is pos- 

 sible only by rock fiowage, through a rupturing and differential movement 

 of the solid particles with reference to one another, or by solution and 

 redeposition — i. e., by granulation or recrystallization, or by the two 

 combined. 



On the assumptions (a) that the strength of the rocks is the same as at the 

 surface, (b) that the rocks are all of the same kind, (c) that the temperature 

 is the same as at the surface, (d) that the water present does not make any 

 difference in the character of deformation, (e) that the rocks yield as readily 

 by fracture as by flowage, (f) that the rocks break as readily by fracture 

 when the deformation is slow as when it is rapid, and (g) that the rocks are 

 among the strongest, I have calculated that the maximum depth of the 

 upper part of the zone of flowage under mass-static conditions can not be 

 greater than 12,000 meters. All of these assumptions, except the first, are 

 in favor of great depth for the zone of flowage. It is explained (Chapter 

 VIII, p. 672) that where rocks are under pressure in all directions the 

 rigidity is probably greater than at the surface. Therefore the assumption 

 that the rocks are no stronger below than at the surface might lead to too 

 small a depth. However, the other assumptions would give too great a 

 depth, because the great majority of rocks are not nearly so strong as the 

 strongest, and many of them have only a small fraction of this strength; 

 because the temperature increases with increase of depth, with orogenic 

 movements, and with intrusives; because water is present in considerable 

 quantity, and where this agent is available with higher temperatures the 

 rocks are deformed by flowage rather than by fracture (see p. 188); and, 

 finally, because the rocks are ordinarily deformed so very slowly that with 

 a rather moderate pressure the deformation takes place by flowage rather 

 than by fracture. I can see no way to determine to what extent these 

 factors render the maximum depth calculated too great; nor can any 

 estimate be made as to how far the factor (a) renders the maximum depth 

 calculated too small; but I suspect that the various factors giving too great 

 a depth are of far greater consequence than the one factor giving too small 



a Winslow, A., An illustration of the flexibility of limestone: Am. Jour. Sci., 3d ser., vol. 43, 

 1892, pp. 133-134. 



