730 A TREATISE ON METAMORPHISM. 



from materials between water solutions and true magmas. During pegma- 

 tization, in proportion as the conditions approach those of a magma, 

 the textures and structures may depart from those of slates, schists, and 

 gneisses and approach those of igneous rocks. Thus there may be all 

 oradations between the textures and structures characteristic of the slates, 

 schists, and gneisses and those characteristic of the igneous rocks. In some 

 regions there occur remarkable combinations of textures and structures 

 characteristic of various rocks; for instance, the clastic rocks, the schists, 

 and the igneous rocks. 



When once a rock, whether a~ sediment, a crystallized igneous rock, 

 or a metamorphic rock, has become a magma by fusion, whether it be 

 by aqueo-igneous fusion or by dry fusion, the resultant material is a true 

 magma. The crystallized rock which forms from it by cooling has all the 

 characteristics of an igneous rock. . The textures and structures formed are 

 those of the igneous rocks. The textures and structures of the fused 

 sedimentary, original igneous, or metamorphic rocks are totally destined, 

 and the newly formed rock is to all intents and purposes a new igneous 

 rock. If rocks derived by the fusion of previous solid rocks have again 

 crystallized on a large scale, it is most difficult to prove this. The contacts 

 between the fused and unfused portions of the material would be similar to 

 those between ordinary intruded and intrusive rocks. The only way which 

 I can suggest to show fusion and recrystallization on a large scale is to 

 prove that the rock supposed to be fused and recrystallized possesses the 

 chemical composition of the rock from which it is believed to have been 

 derived. For instance, a fused and recrystallized shale should possess the 

 textures and structures of an igneous rock, but the essential chemical 

 composition of a sedimentary rock. This criterion of chemical composition 

 as applied to the red granular rock (meta-rhyolite) of Pigeon Point leads 

 to the conclusion that it is probably an independent intrusive, rather than the 

 result of the fusion of the Animikie rocks by the gabbro. a (See pp. 732-733.) 



But while it has not been shown that fusion and recrystallization have 

 taken place on a great scale, various cases have been described in which 

 the evidence seems clear that to some extent intruded rocks have been fused. 



There can be no question that fusion of fragments included within 

 igneous rocks does take place. Very numerous inclusions which are found 



«Bayley, W. S., The eruptive and sedimentary rocks on Pigeon Point, Minnesota, and their con- 

 tact phenomena : Bull. U. S. Geol. Survey No. 109, 1893, p. 118. 



