624 FRANK D. ADAMS AND J. AUSTEN BANCROFT 



plates of kaolin and muscovite lying parallel to one another and at 

 right angles to the direction in which the pressure is exerted. The 

 breaking down of the foliated structure of the rock is indicated on 

 the curves by the irregularities to which reference has been made. 



It will also be seen that in the case of granite, when the lateral 

 resistance is relatively low (e.g., when the rock is inclosed in the 

 steel tube having a 0.25-centimeter wall), there is at the same 

 point a sag, though much less marked, due to the fact that the 

 lateral resistance offered by the tube is not quite sufficient to develop 

 a uniform movement in this the strongest of all the rocks employed 

 in the investigation. 



Attention must be drawn to the manner in which deformation 

 goes forward in a column of rock when deformed under the con- 

 ditions of the experiment. As may be seen, if the tube and the 

 inclosed rock are sawed in two vertically, the column of rock begins 

 to move or flow at the middle, the motion taking place first along 

 the well-known shearing cones, having an angle of approximately 

 45 (usually somewhat greater), seen when a column or cube of the 

 rock is crushed between the faces of a testing machine in the ordinary 

 determinations of the strength of rock for building purposes. Thus, 

 as the movement progresses, there develops within the column two 

 obtuse cones, having as their bases the faces of the advancing pis- 

 tons and consisting of portions of the rock which show no evidences 

 whatsoever of deformation, but which are, under the conditions of 

 the experiment, subjected only to cubic compression. As the 

 experiment progresses, these cones (see A and B in Fig. 8) advance 

 into the deforming rock, additional amounts of the rock shearing 

 off the surfaces of the cones and thus coming to participate in the 

 movements which are going forward. Owing to the fact, therefore, 

 that the quantity of flowing rock is continually increasing in an 

 unknown ratio, it is impossible from the data mentioned above to 

 determine whether the definite increase in the ratio of load to 

 deformation is due to an increase of internal friction developed with 

 increase of pressure, or to the increased amount of material which is 

 being moved. 



The answer to this question is obtained from another series of 

 experiments which exactly duplicated those with the columns of 



