THEORY OF SLATY CLEAVAGE. 67 



If pressure is applied so rapidly that a considerable shock attends 

 nipt me a corresponding quantity of energy will remain in the fragmental 

 mass in t lie form of vibrations. These vibrations will take place along 

 the lines of unaltered direction, making an angle '■ with ox. In the ex- 

 treme ease of scission tins direction is also that of the lines of maximum 

 tangential strain. In every other case the vibrations will occur at an 

 acute angle to the planes of maximum strain, and in no instance will 

 they lie perpendicular to these planes. 



At the instant when the rupture takes place the whole mass is strained, 

 in one or more directions, to the limit of endurance. Rupture and the 

 inception of waves of compression are simultaneous, and these waves are 

 propagated from the surfaces of primary rupture, but not perpendicularly 

 to them. The waves must interfere and. where they intensify one another, 

 there must he resultant shearing couples in the direction of the planes of 

 maximum tangential strain. These waves must be propagated at the 

 same rate that relief of pressure takes place, a rate dependent upon the 

 properties of the mass. If. then, the waves are of considerable ampli- 

 tude, it appears to me that on those surfaces at which they reinforce one 

 another, they must intensify the strain beyond the limit of endurance. 



Thus there seem- sufficient reason to believe that a pressure very 

 rapidly applied, producing primary ruptures attended by shock, will be 

 immediately followed by secondary ruptures in the same direction as the 

 others at intervals dependent upon the wave length of the impulse. In 

 much the same way a high explosive shatters a rock far more than black 

 powder. 



A phenomenon of which no explanation has been offered in this paper 

 is that of thick slates and of those flags which are to be considered as very 

 thick slates. These, though cleavable to a certain thinness, are not capa- 

 ble of further splitting. Such rocks indicate a flow which is not uniformly 

 distributed through the mass, hut, on the contrary, passes through maxima 

 at intervals corresponding to the thickness of a slate or flag. It is pos- 

 sible that at the inception of strain such masses were in a state of tremor 

 so intense that the interference of waves determined surfaces along which 

 flow began. These surfaces would be weakened by the flow, and further 

 strain would be distributed among them rather than over the intervening 

 solid sheets. Effects of a similar kind are produced on a pile of sheets 

 of paper, such as " library slips," resting on an inclined, cloth-covered 

 table which is jarred by rapid blows. 



The question would seem to be one of the direction and intensity of 

 the vibrations rather than of their existence. The tendency of rectilinear 

 motion to pass over into molar vibrations of rapidly decreasing period is 

 so strong as to make it most improbable that such a distortion as is in- 



