MARBLE. 809 



particles into a single mineral. This can be done only by solution and 

 deposition, or recrystallization. It is notable in this connection, as explained 

 below, that in proportion as the rocks were deep seated and as erogenic 

 movements were severe the marbles are likely to be coarse grained, 

 and were therefore thoroughly recrystallized. If the deformation were 

 mechanical, the greater the severity of the movement the greater would be 

 the strain effects mentioned below. 



The development of marble is frequently assisted by the intrusion of 

 igneous rocks. If the masses be large they may furnish the conditions 

 of heat necessary for the solutions to be able to recrystallize the limestone 

 for a considerable distance about the intrusive. Of course, in many regions 

 orographic movements and volcanism are synchronous, and in such cases 

 the conditions furnished by the movements and those furnished by the 

 intrusive combined are particularly favorable to the process of recrystalli- 

 zation. Recrystallization may have been completed in regions of com- 

 paratively little disturbance, and thus occasionally in unfolded regions 

 calcareous rocks are locally found to which the term marble is applicable. 

 But the great belts of marble occur in regions in which orographic move- 

 ments have taken place, and especially in rocks which have been so deeply 

 buried as to be in the zone of anamorphism. This is shown by the asso- 

 ciation of the marbles with slates, schists, and gneisses — rocks which have 

 been metamorphosed in the lower zone. Where the forces and agents 

 were sufficiently potent to recrystallize the more refractory minerals, the 

 recrystallization of the calcite and dolomite is probable, even if the marbles 

 do not give such clear evidence of the fact as does slate, schist, or gneiss. 

 Calcite and dolomite are most ready to respond to stresses by solution 

 and deposition, and therefore to recrystallize, and also mechanically by 

 development of twinning structures, by movement along gliding and 

 cleavage planes, and by granulation. 11 (See PI. VI, C and I).) Indeed, 

 they are the most mobile of all the minerals which make up large masses 

 of rocks. The minerals, halite, niter, etc., are 'even more mobile, but their 

 rock masses are relatively small. 



In consequence of the great mobility of calcite and dolomite, marbles 

 generally differ in structure from the associated metamorphosed slaty, 



«See Adams, F. D., and Nicolson, J T., An experimental investigation into the flow of marble: 

 Philos. Trans. Royal Soc., London, ser. a, vol. 195, 1901, pp. 363-401. 



