A TREATISE ON METAMORPHISM. 



into the openings of the belt of cementation. After a given period of 

 deformation the process of dolomitization continues until the openings are 

 closed and circulation is checked, to begin again as soon as new openings 

 are formed by earth movements. The process goes on with intermittent 

 speed until a state of equilibrium is approximately reached between the 

 calcium-magnesium solutions and the adjacent rock. 



Other excellent illustrations of areas in which the conditions are 

 favorable for dolomitization are furnished by various districts in the 

 Cordilleras. Here in many areas recent orogenic movements and intrusion 

 and extrusion of magma have taken place. The movements open new 

 cracks and crevices in the limestones. The volcanic rocks, intruded or 

 extruded, raise the temperature of the rocks and promote the activity of 

 the solutions. Furthermore, the increase in temperature promotes the 

 process of carbonation in the belt of weathering. In areas where there are 

 limestones, recent volcanism, and orogenic movement, the conditions are 

 especially favorable for the transfer of magnesium from the belt of 

 weathering to the belt of cementation, with resultant dolomitization. 



In describing the processes of recrystallization and dolomitization in 

 limestones, marbles have been accounted for in the main. Marble is the 

 term which is applied to evenly granular, finely or coarsely interlocking 

 calcite, dolomite, or intermediate minerals. (PI. IV, 0.) In the process of 

 recrystallization it is common for a change to take place in the color of the 

 rocks, as a result of which marbles are ordinarily white or pinkish white, 

 rather than blue* or gray, although marbles may, of course, have various 

 other colors. 



As already mentioned, marble results mainly from limestone in conse- 

 quence of recrystallization or dolomitization, or the two combined. The 

 absolutely conclusive evidence on this point is the increase of size of the 

 mineral particles in marbles -as compared with limestones. (See PI. VI, A 

 and B.) The formation of one mineral particle of marble of average size 

 required a multitude of particles of the limestones, probably thousands; 

 and in those cases where the grains of marble are coarse and the original 

 limestone was fine perhaps many thousands were required. There is no 

 mechanical process which alone can merge a great number of mineral 



