Van Hise — Metam orphism of Rocks and Rock Flowage. 91 



tallizes according to the laws of magmas, and produces 

 textures which are characteristic of such crystallization, and 

 these are widely different from those of the crystalline schists. 

 It does not follow from the foregoing that the deeply buried 

 rocks, including the crystalline schists themselves, may not be- 

 come modified or even fused by contact with igneous 

 intrusives. 



Summary of Conclusions. 



I here repeat some of the more fundamental principles 

 stripped of qualifications : 



(1) The chemical alterations which rocks undergo vary 

 greatly under different conditions. The more important of 

 these variable conditions are water content, temperature, pres- 

 sure, and movement. 



(2) The outer part of the earth, of which we have definite 

 knowledge, may be divided into two physico-chemical zones. 



In the upper of these the reactions take place with the ex- 

 pansion of volume and with the liberation of heat, as end 

 results. In the lower the reactions take place with the con- 

 traction of volume and with the absorption of heat, as end 

 results. Some of the more important reactions in the upper 

 zone are hydration, oxidation, and carbonation ; some of the 

 more important reactions in the lower zone are dehydration, 

 sulphidation, and silication. 



(3) The alterations under mass static conditions preserve 

 previous textures and structures, but may go so far as to com- 

 pletely recrystallize the rocks. The alterations under mass 

 dynamic conditions are different in the zone of fracture and 

 the zone of flow. In the former the rocks are broken into 

 fragments, and the alterations of the fragments are those of mass 

 static conditions. In the zone of flow the alterations obliter- 

 ate previous textures and structures and produce crystalline 

 schists which have characteristic textures and structures. 



(4) Rock flow is accomplished partly through mechanical 

 strains, but mainly through continuous solution and deposition 

 of the material of the rocks by the agency of the contained 

 water. During the flow the rock is at all times almost wholly 

 a solid, yet it responds like a plastic body to deformation 

 without loss of its crystalline character, because of the con- 

 tinuous adaptation of the mineral particles, while in large 

 parts retaining their integrity, to new forms by recrystalliza- 

 tion. 



(5) The energy required to produce a given mass deformation 

 increases downward to the bottom of the belt of granulation. 

 In the zone of flow by recrystallization the energy required 

 to produce a given mass deformation is less, probably 

 much less, than that in the lower part of the zone of fracture. 



