DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 513 



them, so that, with the lateral expansion or bulging of the cylinder, 

 there is combined a tendency for a portion of the cylinder to move 

 more rapidly along some one plane, developing a specially pronounced 

 shear in this direction. No rupture ensues, but the more pronounced 

 movement in this direction is evident from the form of the distorted 

 cylinder. This is shown in Plate IV, Fig. a, where the traces of 

 such planes bound a V-shaped projection on the front of the 

 deformed column. On the base or end face of the bulging portion 

 of the cylinder, the lines above described are not seen, unless they 

 be represented by a series of little, somewhat irregular radial fissures 

 noticed where the deformation is very pronounced. 



One noteworthy fact observed in every case where a cylinder of 

 the marble was deformed in parafl&ne is that the column on removal 

 of the parafhne is found to be cracked or fractured transversely, that 

 is, in a direction at right angles to the axis of pressure. This is seen in 

 Plate IV, Fig, a, where two of such fractures parallel to one another 

 were developed in the same cylinder. The surface of such a fracture 

 plane is approximately flat but not absolutely smooth or polished, 

 and in partially deformed cylinders it frequently occurs just about 

 the line between the deformed and undeformed portion. The same 

 planes of transverse fracture are developed, upon the relief of 

 pressure, in very strong fine-grained limestones when they are 

 deformed in steel tubes. It is apparently connected with the elastic 

 expansion of the rock on the removal of stress. 



The invariable presence of this transverse fracture makes it 

 impossible to determine the strength of the deformed column in com- 

 pression. While probably not so strong as the original marble, it is 

 still firmly coherent and hard, withstanding a sharp blow without 

 breaking. When the deformation is pushed to an extreme, in addi- 

 tion to the Luder's lines, a series of faint, slightly wavy lines, running 

 in a horizontal direction across the column and hence in a direction 

 at right angles to the pressure, is developed. 



When a section of the deformed marble is examined under the 

 microscope, the decrease in transparency of the rock as compared 

 with the original marble at once arrests the attention. This, on close 

 examination, is seen to be due to the development of an immense 

 number of twinning lamellae in the constituent calcite grains, often 



