LIMITING STRENGTH OF ROCKS UNDER STRESS £37, 
§ 10. SUMMARY AND CONCLUSIONS 
1. Secs. r to 3 are devoted to a mathematical discussion of the 
state of stress existing in cylindrical rock specimens incased in 
heavy nickel-steel jackets and described by Dr. Adams in an accom- 
panying paper. 
2. In sec. 4 it is shown that observations on the radial strains 
of the nickel-steel jacket agree well with the calculated values as 
long as the load does not exceed a certain value; beyond that 
point the results of experiment indicate a change in the elastic 
constants with load; an analysis of observations of this type could 
be made to give a relation between the rigidity and the load for 
stress-differences beyond the elastic limit. The results of sections 
4 and 5 also give a good account of the manner in which the rock 
specimens break down under load. 
3. The experimental results of Dr. Adams discussed in sec. 6 of 
the present paper show that in the neighborhood of a small cylin- 
drical cavity, in a specimen of Westerly granite, no flow or set 
takes place when the stress-difference amounts to as much as 160,- 
ooo pounds per sq. in. The cavity just begins to break down for a 
stress-difference of 200,000 pounds per sq. in. We therefore assign 
to the limiting stress-difference in the neighborhood of small cavities 
in rocks typical of that forming the greater part of the earth’s crust, 
a value S between 160,000 and 200,000 pounds per sq. in. for 
ordinary temperatures. 
4. Secs. 7 to 9 deal with the existence of cavities in media under 
stress with especial reference to the depth at which cavities in the 
earth’s crust can remain open. 
It is shown that no state of shearing stress in the crust of the 
earth, due to the weights of continents and mountains can cause 
the collapse of the rock in the neighborhood of a small cavity. 
It is also shown that as far as hydrostatic pressure in the earth’s 
crust is concerned a small cavity at ordinary temperatures will 
remain open provided the depth does not exceed a value between 
17.2and 20.9 miles. Ata temperature of 550° C. supposed to exist 
tz miles below the earth’s surface, cavities will remain open when 
submitted to considerably greater pressures than are found at this 
depth. These values greatly exceed previous estimates because 
