902 PROCEEDINGS OF THE ACADEMY OF [DeC, 



expelled entirely had the blow been a little harder. In this case, how- 

 ever, another rim of crushed and shattered rocks would have been 

 upraised around the enlarged hole. 



Comparison of the Crater with those Produced by Lesser 



Projectiles. 



The craters formed by the impact of various small projectiles, mostly 

 of soft materials and at low velocities, have been studied in connec- 

 tion with the formation shown in this locality by others, notably by 

 Professor Gilbert, and the forms shown to bear a rather close resem- 

 blance to the crater of Coon Butte and its rim. Continuing these com- 

 parisons, however, to more violent impacts of heavier bodies at higher 

 velocities, a still closer parallel is noticed. The material for such 

 comparisons is furnished by the investigations of the several more ad- 

 vanced military nations upon the effects of the impact of round shot on 

 masonry and solid rock. These investigations were undertaken about 

 sixty to seventy years ago, with the object of ascertaining the best 

 effects of the ordnance of that day in the breaching of walls, etc., in 

 bombardments. The general result was to establish the fact that the 

 impact of the projectile produced a comparatively shallow crater of 

 conical form about five times the diameter of the projectile, termi- 

 nating in an almost cylindrical hole some one and a half to twice the 

 diameter of the projectile within which the projectile or its wreck 

 was deposited. This hole was surrounded by a cone of broken and 

 shattered material which started at or below the bottom of the cylin- 

 drical hole and enveloped the actual cavity. The depth in solid lime- 

 stone and sandstone, at velocities at which the best cast iron shot would 

 break up, and estimated, from the powder charges used, to be somewhere 

 about 1,800 feet per second, was a fraction under two diameters of the 

 projectile used. The depth was observed to increase much more 

 slowly than the velocity of the shot, and more slowly still after the 

 velocities at which the shot would break up had been attained. The 

 author has observed from direct experiment that the crater still retains 

 its round form even when the impact of the projectile is as far removed 

 from the vertical as twenty degrees ; the only noticeable effect being 

 the greater shattering of the side of the crater against which the angle 

 of impact caiLses the projectile to bear with most pressure in its 

 penetration. These experiments were made with a high power, 

 small-bore rifle, having an initial velocity of about 2,300 feet per 

 second. 



