ARTIFICIAL SLATY CLEAVAGE. 



81 



If a bar were substituted for a wire, and slots for the circular openings 

 of the draw-plate, the strain would be exactly equivalent to that pro- 

 duced by an inclined pressure acting on a rigidly supported cube. It 

 cannot be doubted that in such a case the end of the bar would also 

 become concave, and that evidences of schistosity would appear. 



When a plastic mass is extruded through a small opening, whether 

 circular or rectangular, the action is very similar to that involved in 

 drawing wire, excepting that the external force is a pressure instead of a 

 tension. The friction on the moulding surface delays the motion of the 

 external layers relatively to the internal layers, and so-called "fluxion 

 structure results. In the following diagram it is plain that a cube of the 

 plastic mass at a would become an oblique parallelopipedon at 6* 



Fiouke 10. — Development of Cleavage by Extrusion. 



When an oblique blow is struck with a pick the bruise will manifestly 

 show a distortion of a very small cube similar to those already considered. 



The case of a direct pressure, such as was employed by Professor Tyn- 

 dall, seems at first sight very different from the foregoing. To convince 

 myself as to the mechanics of the matter I repeated his experiments, 

 with the following results : f A cake of wax can be compressed to less 

 than half its thickness between glass plates well greased with a heavy 

 oil without bulging of the edges, as shown in figure 20, a, b. If such 

 cakes are cooled to — 15° C. they show no slaty cleavage, but exhibit a 

 tendency to split at an angle of some 60°, more or less, to the line of 

 pressure. If the plates are not greased, hut only wet with water, as in 

 Tyndall's experiments, there is ;i strong tendency to bulge along the 



*M Daubr6e, in his Geologie Experimentale, r. mm. ids striking experiments on this mode of de- 

 formation. 



f White wax is I irtter than yellow for the purpose of this experiment. To get comparable masses 



least cylindrical cakes at as low a temperature as practicable. These wen led off and then 



kept iu water at about 40° C. for an hour or more. I', -low this temperature the wax is boo hrittle to 

 mould with ease or rapidity. The compressed cakes were cooled in ice and salt. Cakes chilled 

 without preliminary distortion show no cl savag • under the hammer or chisel, and Crack \ ery like 

 fine-grained basalt. 



