and the Elastic Limit of Lama. 291 



upon a volcano is 40° upon Kumagatake, while he notes in 

 pictorial representations of volcanic cones angles often exceed- 

 ing 50° and one reaching 69°. He is inclined to think that 

 artistic feeling may have induced exaggerations in these cases. 

 Other geologists- have of course also called attention to such 

 misrepresentations. If my theory of the form of volcanic 

 cones is correct these objections are well founded, since 45° is 

 reached only at the (impossible) point of a solid cone. If the 

 thin walls of large craters, however, are sufficiently solid to 

 take a form of least variable resistance higher angles than 45° 

 will occur. 



The elastic limit of the average lava of cones. — Besides its 

 geometrical importance in the equation of the volcanic cone 



2k 



the quantity — possesses a further property of at least equal 



P 

 interest. The coefficient of resistance at the elastic limit of 

 the material of the cone is 



P 



u = —C. 



2 



Now x is a constant which it is peculiarly difficult to deter- 

 mine experimentally for any material, while it is one of prime 

 importance in the grand question of geology, upheaval and 

 subsidence. The value of c, however, can be immediately 

 derived from observations on volcanic cones or from drawings 

 to scale or from photographs of which the scale and the angle 

 of the plane of projection to the vertical are known, while p is 

 determinable for uniform material with the utmost accuracy 

 and ease and a close approximation to its average value could 

 doubtless be obtained by a considerable number of experi- 

 ments for the materials of almost any volcanic cone. The 

 value of p is capable of being further checked by the results of 

 pendulum observations. The form of the more regular class 

 of volcanic cones will therefore enable geologists to determine 

 the modulus of resistance for the elastic limit on an enormous 

 scale for an extremely important class of the constituents of 

 the earth's "crust," and if the scale should prove not to com- 

 pensate for the uncertainty as to the value of the density, the 

 method cannot fail to afford a valuable check on those ob- 

 tained from laboratory experiments. 



Mr. Mark B. Kerr of the TJ. S. Geological Survey has kindly 

 furnished me with a surveyed section of the " Sugar Loaf," 

 Siskiyou County, California. It is shown in fig. 2 with the 

 theoretical curve on a larger scale than that employed in fig. 1. 

 The agreement is very good and the scale being known gives 



c= — =2560 feet= 780-26 meters. 

 P 



