THE STRENGTH OF THE EARTH'S CRUST 437 



for steel — 



«/ = 829X io''(i — . 000,187/— . 000,000,59/^+ . 000,000,000,9/5). 



This equation for iron gives a minimum modulus at 314° C. equal 

 to 95 per cent of the modulus at 0° C. For steel the minimum 

 value occurs at 342° C. and is 90 per cent of the modulus at zero. 

 At 528 °C. iron has the same modulus as at 0° C. and at 890° C. steel 

 has the same modulus as at zero. Doubtless 1000° C. is above the 

 limits of the data from which these formulas were derived. For 

 this temperature they may consequently give inaccurate results, 

 but it is of interest to note that the curve gives a modulus of 

 rigidity for iron at that temperature i . 7 times that at 0° C. and 

 for steel i . i times that at 0° C. The extrapolation prevents 

 attaching quantitative value to these figures, but the quahtative 

 conclusion may be reached that iron and steel at high temperatures 

 do not exhibit less rigidity than they possess at lower temperatures. 

 It is obvious, however, that above a certain temperature the elastic 

 limit becomes very low, as shown by the capacity for forging, 

 and for strains beyond this limit deformation takes place by 

 plastic flow. That it is not merely incipient fusion is suggested 

 by the maintenance of a crystalline condition through the process 

 of deformation. The subject for iron is doubtless complicated by 

 the fact that iron passes through more than one solid molecular 

 state in being heated up to fusion. Presumably then the equation 

 given for the relation of rigidity to temperature can only be a first 

 approximation to the actual changes. 



Let the attention be given next to the crystalHne rocks which 

 were once deep-seated and, owing to subjacent bathoHthic invasion, 

 attained their crystallization at exalted temperatures. It is ob- 

 served that, although the rock masses have been extensively 

 deformed, the individual crystals have regrown during the process 

 so as to possess compact boundaries, and an internal constitution 

 nearly free from strain. The interpretation is that the deforma- 

 tions due to geologic forces were so slow and the rocks were so 

 saturated with crystallizing agents at high temperatures that 

 recrystallization could nearly keep pace with the deformation, even 

 for temperatures below the range of plasticity. As understood by 



