GENERAL STATE OF STRAIN IN ROCKS. 599 



the rocks are much strained, by that process it is largely relieved from 

 strain, and a slide of it placed under the microscope may show little or no 

 evidence of its former state of strain. 



It therefore appears probable that a high state of strain is the common 

 condition for the great mass of the rocks constituting the belt of cementa- 

 tion. It has been explained on pages 95-98 that in proportion as min- 

 eral particles are in a state of strain they are likely to be acted upon 

 chemically by the ground solutions. Therefore the very general and 

 marked state of strain in the rocks of the belt of cementation is probably a 

 very important factor in their alteration. This is especially true of meta- 

 somatism. By that process, as explained on pages 690-692, the rocks 

 may be released from strain, the mechanical energy producing the strain 

 being utilized in promoting recrystallization. It is difficult or impossible 

 to prove the importance of state of strain in metasomatism except by 

 general reasoning, based upon the experimental facts referred to in Chapters 

 III and VIII. But it is my confident belief that the general state of strain 

 is one of the most important of the inciting causes of alteration. 



STRAIN BEYOND ELASTIC LIMIT. 



Strain beyond the elastic limit results in fracture. The belt of cemen- 

 tation belongs to the zone of fracture. It has been seen in the previous 

 sections that deformation may be accomplished to a considerable extent by 

 differential movement of the particles of incoherent rocks and by strain 

 within the elastic limit, Where the rocks are coherent marked deforma- 

 tion in the belt of cementation is largely accomplished by fractures and 

 by movements along the fracture planes. In the very partially indurated 

 rocks fracturing may consist in the rupturing of the particles from one 

 another, and adjustment come by differential movements between the par- 

 ticles. But in the coherent rocks the ruptures are wider spaced than the 

 individual grains. In the weaker coherent rocks the surfaces of rupture 

 are more or less irregular, ordinarily breaking around rather than cutting 

 through the grains. This form of rupture is well illustrated by sandstones. 

 The strongly coherent rocks usually show clean-cut fractures which cut 

 the mineral particles. 



When rocks in the belt of cementation are strained beyond the elastic 

 limit there is a marked tendency for the ruptures to follow planes more or 



