900 PROCEEDINGS OF THE ACADEMY OF [DeC, 



way in which such substances yield to either a pressure or blow in 

 excess of their power of resistance is, that a cone of material with an 

 apex angle of about ninety degrees is compressed downward into the 

 solid mass of the material from the point of impact. This cone parts 

 from the overlying material, crushes into powder under the force of 

 the pressure or blow, and this powder being still further compressed 

 transmits the pressure upon it in all directions, somewhat like a 

 fluid, although not equally in all directions. The pressure thus gen- 

 erated in the very substance of the material seeks relief and forces a 

 yielding of the solid material around it, which, of course, occurs along 

 the line of least resistance, and bursts the surface upward and outward 

 into a cone-shaped crater around the point of impact or pressure, the 

 angle of which depends largely upon the nature of the material. With 

 ordinary stone this is usually about thirty degrees, but always must be 

 less than forty-five degrees, which is its limit. This crater-like cone 

 is small at first and remains so for weak impacts or small pressiu-es, 

 but if these are greater the process is continued by the formation of 

 larger cones of compressed powder, deeper in the bod}' of the material, 

 which relieve themselves by bursting up wider craters, until the force 

 of the pressure or impact is no longer able to continue the process and 

 the penetration ceases. Thus the depth of the crater always bears a 

 definite relation to its width, and in large impacts it is found that the 

 crater is always surrounded by a cone of cracked and shattered material, 

 which would have been the next material to be expelled if the energy 

 of the blow had been sufficiently great to accomplish this. 



The bearing of this upon the formation of a rim composed in part of 

 fine powder is as follows. The broken rocks and debris that are ex- 

 pelled from the hole get their velocity imparted to them by the push 

 of an inelastic powder behind them and not by a compressed elastic 

 gas, and thus when both rock fragments and powder have progressed 

 far enough to free themselves from the pressure of the penetrating 

 projectile they fly on together, mixed powder and rocks, at the same 

 velocity. This powder is not dust in the ordinary acceptation of the 

 word, as fine powder mixed with a large quantity of air which takes a 

 long time to settle out, but is almost unmixed with air in solid masses, 

 particle to particle, like flour in a barrel, so to speak, which masses 

 obey the laws of projectiles and falling bodies, irrespective of the ex- 

 ceedingly minute particles of which they are formed, and are thus de- 

 posited in the rim in mixture with and under and over the solid rock 

 masses which accompanied it in its flight, and as quickly ; and the powder 

 having started under the rock masses, there is a strong tendency for con- 



