300 b. willis discoidal structure of the lithosphere 



Explanation of Plates 



Plate 8. — Curves of ahsolute Strength and relative Strength of Rock from the 

 Surface to 100 Miles helow it, at surface Temperature 



The curves in plate 8 represent hypothetically the absolute and the relative 

 strength of granite at surface temperature, but at depths ranging from the 

 surface down to lOO miles below it. 



The absolute strength is deduced from Adams' experiments on the increase 

 of internal friction or crushing load under high pressures. The result is rep- 

 resented by the abscissas of the curve LMNPQR. 



L is a point on the surface. The strength is equivalent to a crushing load 

 of 5 miles of granite. M and N are points determined by Adams' experiments 

 on cylinders of granite inclosed in yielding steel jackets, which gave for a 

 confining pressure equivalent to burial at 4.2 miles below the surface a strength 

 equivalent to a load of 21.6 miles, and similarly for 5.8 miles burial a crushing 

 load of 23.3 miles. P is a point deduced from Adams' experiment on a per- 

 forated granite cylinder inclosed in an unyielding steel jacket. From P to R 

 the curve is hypothetically extended, Q being adopted from a study of Adams' 

 experiments as the probable depth at which the strength would equal the load, 

 40 miles. 



The curve XYZ expresses the loss of strength due to rise of temperature, on 

 the assumption that melting reduces the internal friction of the rock to zero, 

 which is only approximately true. The curve has no quantitative value. 



The relative strength of granite, as compared to the load, is shown by the 

 curve RST and its prolongation. The abscissas of the curve, plotted to the 

 left of the axis, represent the ratio of the crushing strength to actual load, 

 Wc/Wa. This may be called the factor of safety. At the surface it is infinite. 

 At the depth of 4.2 miles it is equal to 21.6/4.2 or 5.1. Similarly, at 5.8 miles 

 it is 4. At 40 miles it is by deduction equal to 1. Below that depth it is a 

 fraction, but it can not reduce to zero unless the temperature be raised to the 

 melting point. Pressure alone can only increase the absolute strength, even 

 though the relative strength becomes less than 1 — that is, at uniform tempera- 

 ture the absolute strength increases with increasing pressure, even though the 

 rock is potentially crushed. 



Plate 9. — Diagram of Compression and thermal Expansion of Rock 'between 

 the Surface and a Depth of J^O Miles 



On the right and left of plate 9 are shown depths and corresponding pres- 

 sures and temperatures in the lithosphere down to 40 miles. Pressure and 

 temperature also are expressed by the straight lines so marked, according to 

 the scale at the bottom of the diagram. The rates of increase accepted are 

 those adopted by Adams (specific gravity, 2.8; temperature increase, 1° C. for 

 every 32.9 meters). 



The diagram is designed to express the antagonistic effects of pressure and 

 heat on the volume of rocks. Plate-glass is taken as a representative of rocks 

 in its behavior toward compression and expansion. The curve marked "Hypo- 

 thetical compression" represents the supposed effect of pressure in reducing 

 the volume of a rock at uniform temperature. The curve marked "Hypo- 



