DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 523 



however is, as has been mentioned, that of a stiff paste, not that of a 

 brittle solid. 



F. IMPURE MAGNESIAN LIMESTONE: HULL, CANADA 



This is a very impure magnesian Kmestone, containing about 50 

 per cent of insoluble residue in the shape of minute subangular grains 

 of clear quartz. The rock is of somewhat open grain and porous 

 character and has been used very extensively for the production of 

 hydraulic cement. 



When deformed in paraffine, triangular portions of the column were 

 found to have sheared off around the end, as in the case of other 

 purer varieties already described. These remain adhered to the 

 column, which also shows a marked tendency to develop cracks, 

 crossing it at right angles to its length, i.e., at right angles to the direc- 

 tion of the pressure. 



A column of the usual dimensions was then deformed in alum 

 in the usual manner under a load of 31,000 lbs. On dissolving 

 away the alum, however, the deformed column went to pieces, but 

 from the shape of the fragments it could be seen that the movements 

 developed in it had been of the nature of a complicated shearing 

 similar to those already described. 



G. BIOTITE granite: BAVENO, ITALY 



Columns of granite of the usual size and with a polished surface 

 were embedded in alum inclosed in a copper tube in the usual manner 

 and submitted to the pressure required to squeeze the whole down 

 until the copper tube displayed signs of incipient rupture. The 

 load required for this purpose was 50,000 lbs. 



When the alum was removed by solution, the granite at one end 

 of the column was found to have remained intact. Toward the 

 middle, however, the column had undergone a distinct bulging due 

 in part at least to a movement along little planes of thrust or shearing, 

 although the work was still quite coherent, the movement in question 

 having given rise to a rude gneissic or schistose structure owing to 

 the arrangement of strings of mica and grains of orthoclase parallel 

 to the base of the column. The other end of the column where the 

 motion had been greatest was disintegrated by the movement and fell 



