752 A TREATISE ON METAMORPHISM. 



tho continuous solution and deposition of calcium carbonate in the chemical 

 laboratory when water is passed through a layer of this material under 

 pressure. Where the pressure is greatest, in the upper part, the grains are 

 taken into solution; at the place of escape, where the pressure is least, the 

 material is deposited from solution and the grains increase in size, or grow. 

 During the deformation of the rocks this process of solution and 

 deposition of the mineral particles is continuous. 



If it be supposed to go on to a stage in which an original particle is 

 one-half or one-third as thick as it was originally, it is not necessary to 

 suppose that the central part of the mineral particle has been recrystallized. 

 This is illustrated by fig. 20. A spherical grain is supposed to have changed 



to the superimposed spheroidal grain. The 

 common portion C, or a large portion of it, 

 may be an uncrystallized part of the old grain, 

 but the material AA has been dissolved and 

 added to the borders at BB. Corresponding 

 to this explanation, some of the flat quartz 



Fig. 20,-The possible relation of old and new orainS of the slates and Schists of the Black 



grains of recrystallized rocks. ° 



Hills of Dakota show residual cores." 

 Since in most cases the stresses are unequal in three dimensions, there 

 is a direction vi greatest pressure; at right angles to this is a direction of 

 mean pressure ; and at right angles to these two, a direction of least pressure. 

 During the process of recrystallization, at any given moment there is maxi- 

 mum shortening- in the direction of greatest stress, maximum elongation in 

 the direction of least stress, and shortening or elongation in the direction of 

 mean stress. Consequently the shape of the modified particle may be that 

 which would be produced if a plastic grain were rolled out, the sides being 

 confined in one direction, but with liberty to elongate in another direction 

 in the same plane; or it may be that which would be produced if a roundish 

 cake of dough were flattened between two boards, and consequently elon- 

 gated in all directions at right angles to the direction of greatest pressure; 

 or, finally, the mean stress may approach so closely to the maximum 

 stress that there is shortening in two directions and elongation in a single 

 one only, in which case a fibrous, structure is produced. But from my 



"Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1, 

 1890, p. 224. 



