FACTS CONCERNING RECKYSTALLIZATION. 689 



the typical schists to the random orientation of some of the minerals in the 

 same rocks there are gradations; also there are gradations from the schists 

 recrystallized under mass-mechanical conditions to rocks recrystallized 

 under mass-static conditions where none of the minerals show a marked 

 tendency to similar crystallographic orientation. 



In many cases the similar orientation of mineral particles in a typical 

 schist or gneiss may have been greatly disturbed by subsequent deformation 

 near the surface, and therefore in the zone of fracture. Under such 

 conditions shearing fractures may be produced parallel to the slatiness or 

 schistosity, and the shearing motion between the layers may largely destroy 

 the original regularity of the orientated particles. 



The particles of some of the mineral constituents of igneous rocks 

 which have not been recrystallized show a tendency toward parallel crys- 

 tallographic orientation. With this structure are other structures charac- 

 teristic of rocks crystallized from a magma. I know of but few instances 

 where unaltered igneous rocks so closely resemble the recrystallized schists 

 and gneisses that there is great trouble in distinguishing them. 



In the production of the characteristic textures and structures of the 

 slates, schists, and gneisses, the original textures and even the structures 

 may be destroyed, whether they be those of sedimentary or those of 

 igneous rocks. In passing from an area metamorphosed under mass-static 

 conditions to an area altered during mass-mechanical action, often all stages 

 of destruction of the original textures and structures and the development of 

 new textures and structures may be seen. In an intermediate stage the 

 larger particles or more refractory minerals may show the textures of the 

 original rock, the matrix of the same rock, however, having the texture of 

 a slaty or schistose rock. In instances of extreme alteration under mass- 

 mechanical conditions no trace of the original textures remains, even where 

 the rocks were coarse conglomerates or coarse porphyritic, igneous rocks;' 

 and the secondary structures may traverse the directions of the original 

 structures and the latter may be wholly obliterated. 



Thus metamorphism under mass-mechanical action stands in sharp con- 

 trast to metamorphism during mass-static conditions, in so far as textures 

 and structures are concerned. In metamorphism during mass-mechanical 

 action there is a tendency to destroy old textures and to produce a charac- 

 teristic texture, the more important features of which are mineral particles 



MON XLVII — 04 44 



