254 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



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Before Compression 



made possible to ascertain the internal friction by subtracting the resist- 

 ance of the steel cylinder from the total pressure. 



To apply these results to the case of the lithosphere, let us consider the 

 pressure applied to the ends of the rock specimen as a tangential or hori- 

 zontal stress, and conceive the resistance of the steel cylinder to be 

 replaced by a vertical rock column which would exert the same pressure. 



Then, in order that the latter 

 should be raised in the manner and 

 to the degree that the steel cylin- 

 der was bulged, the differential 

 horizontal stress must be sufficient 

 to overcome, not only the weight 

 of the rock column, but also the 

 internal friction or flow-strength 

 of the rock specimen. 



Adams used steel jackets having 

 walls 0.25 centimeter thick in one 

 set of experiments, and in a sec- 

 ond set 0.33 centimeter thick. The 

 former exerted a resistance of 

 36,685 pounds to the square inch, 

 and was therefore equivalent to the 

 load of 4.2 miles of rock of specific 

 gravity 2.8. The steel cylinder, 

 0.33 centimeter thick, resisted with 

 a pressure equivalent to 37,359 

 pounds, or a load of 5.8 miles of rock. 



The above values are those given by Adams for the stage of experiment 

 ^Svhere the regular column shows a diametrical increase of 0.05 inch 

 (or 6.35 per cent), that is, when the deformation is well under way, and 

 after which it becomes proportional to the tangential pressure.'' Plotting 

 the numerical results given by Adams for successive stages of the experi- 

 ments on granite, one obtains curves which show very marked deforma- 

 tion before the steel had developed a resistance equal to the values given 

 above, and if one were looking for the precise point at which the rock 

 began to flow, somewhat less depths than 4.2 and 5.8 miles would prob- 

 ably be chosen; but I prefer to take the values deduced by Adams as 

 representing a state of established flow. 



Granite confined in a steel jacket 0.25 centimeter thick flowed under 

 a total pressure per square inch equivalent to a column of rock 25.8 miles 

 high, the total being made up of two parts, namely, 4.2 miles correspond- 



After Compression 



Figure 1. — Longitudinal Section through 

 steel Cylinder with Wall 0.33 centimeter 

 and inclosing one of the Rock Columns. 



