102 FRANK D. ADAMS 
ated by. him as not having been taken into consideration, three are 
manifestly of much greater importance than the others in their 
bearing upon the depth of the Zone of Flow. These are: (1) the 
influence of pressure upon the rigidity of rocks in the deeper parts 
of the earth’s crust; (2) the existence and influence of the higher 
temperature found at such depths; (3) the effect of long continued 
pressure as compared with pressure applied for short periods. 
The other factors referred to are apparently of less importance. 
Thus the softer rocks—sandstone, limestone, etc.—occurring in 
the earth’s crust do not extend to any considerable depth; the 
existence of water in the deeper parts of the earth’s crust is a matter 
about which there is still a considerable amount of doubt; while 
it would seem that the strength of granite may be taken as fairly 
representative of that of the stronger rocks of the relatively 
superficial portion of the crust. 
The question then arises as to whether it is possible, in approach- 
ing the question of the depth of the Zone of Flow from an expe- 
rimental standpoint, to reproduce the conditions of rigidity, 
temperature and long continuance of pressure which exist in the 
earth’s crust and thus give due value to these three hitherto doubt- 
ful factors in the problem. It would seem that this can be done 
approximately at least in the case of the first and second of these 
factors, but that it is practically impossible to accurately repro- 
duce the third experimentally, for it is impossible in an experi- 
mental research to extend.the duration of an experiment over 
decades or centuries. But even here some approach can be made 
to the reproduction experimentally of the conditions of this third 
factor, in the first place by extending the duration as much as 
possible, and secondly by the substitution of a relatively higher 
pressure during the shorter period within which the experiment is 
necessarily confined for the lower pressure existing in nature for 
longer periods, since a higher pressure during a shorter time will 
approximate at least in its results to the effects produced by a lower 
pressure for a longer time. 
Before the attempt is made to reproduce the conditions of 
pressure and temperature which obtain within the earth’s crust, 
it is necessary to form as clear a conception as possible as to what 
