DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 521 



D. BLACK marble: BELGIUM 



This is the well-known ornamental stone which in commerce is 

 known as "Belgian Black" or "Noir Fin." It is an impure, some- 

 what bituminous limestone, which is impalpably fine in grain, 

 breaking with a splintery fracture like glass, and which takes a very 

 high polish and is extensively used for interior decoration. . 



When thin sections are examined under the microscope the rock 

 is found to be so fine in grain that a high power is necessary to resolve 

 it. It is composed of minute calcite grains from 0.02 mm. to 

 0.002 mm. in diameter and of irregular shape, between and around 

 which are occasional minute films and spots of a black color. 



When submitted to compression in paraffine it gave rise to forms 

 identical with those developed in the Solenhofen limestone under 

 the same conditions. 



When sulphur was employed as an embedding material, the ends 

 of the column were found to have been forced into the central portion, 

 with the consequent development on the exterior surface of a most 

 complicated series of little tongues or wedge-shaped portions of the 

 rock, sheared up one upon the other like overlapping shingles, thus 

 giving rise to a corresponding increase in the thickness of the deformed 

 column as its height is reduced. The rock after deformation remained 

 hard and solid; it could be rapped sharply on a table without break- 

 ing. The cohesion may have been due in part to a little sulphur which 

 had soaked into the column, acting as a cementing material. No 

 trace of sulphur, however, could be detected on the surface of the 

 column, the heat to which it was subjected after the sulphur had 

 melted and drained away having entirely volatilized any portion of 

 that substance that still remained adhering to the rock. 



When alum was employed, the surface of the deformed column 

 was found to be covered with a great number of sharp and more or 

 less wedge-shaped pieces, often separated by minute open cracks, 

 of which a great number traverse the column. The end faces of the 

 column were also divided into separate areas, often separated by open 

 cracks, which areas form the bases of wedges which have been faulted 

 up or down. The resulting form is similar to that obtained when 

 the rock is deformed in sulphur. The little wedges showed no 

 evidence of plastic deformation. The rock was broken in a marvel- 



