50 A TREATISE ON METAMORPHISM. 



and wide apart, as in the case of faults; or of moderate extent and width, 

 as in the case of joints and bedding partings; or close together, as in the 

 case of fissility. The irregular ruptures may be continuous and the open- 

 ings wide, as in the case of the coarse breccias; or discontinuous and the 

 openings small; or so minute as to affect the individual particles, and thus 

 grade into deformations without openings or granulation. 



Under natural conditions, in order that the pressure shall not exceed 

 the crushing strength of a rock in all directions, it is necessary that it shall 

 be in the zone of fracture for that rock. 



Permanent strain with closing of openings and welding. Mechanical action may d0Se 



openings in rocks and weld the separated parts. In this case there is a 

 diminution of volume due to bringing the particles closer to one another. 

 In order that welding shall take place there must be sufficient pressure in 

 all directions to bring the particles so close together that the molecular 

 attractions are effective, or the pressure in all directions must be greater 

 than the crushing strength of the rock. 



Of course, the pressure required to satisfy the above conditions lor 

 welding depends very greatty upon the character of the material. Moder- 

 ate pressure may be sufficient to weld material composed of small and 

 weak particles. For instance, moderate pressure of clay may bring many 

 of the minute particles of kaolin so close to one another as to place them 

 within the limits of effective molecular attraction. When the clay is dried 

 the mass becomes harder. This hardening is doubtless due in part to the 

 precipitation of the dissolved material contained by the water and the conse- 

 quent cementation of the particles, as explained on pages 617-621. In pro- 

 portion as the particles are coarse, strong, and large, and have relatively 

 few points of contact, the pressure necessary to produce welding increases. 

 To produce deformation with welding of the separated large particles of 

 the strong minerals considerable pressure is necessary. 



WATER ACTION. 



The movement of water under the force of gravity is of the utmost 

 importance in metamorphism. It is, indeed, the great agent of transporta- 

 tion of material both overground and underground, and is the dominating 

 agent through which metamorphism is accomplished. Its work is fully 

 considered in Chapter III, on "The agents of metamorphism." 



