METAMORPHOSED SEDIMENTARY AND IGNEOUS ROCKS. 911 



reciystallized and the larger particles, being more resistant, only flattened. 

 Thus schists derived from porphyritic igneous rocks and from sediments 

 containing materials of varying coarseness may closely resemble each other. 

 Another way in which close resemblance between porphyritic schists 

 of sedimentary and of igneous origin may arise is the development of 

 porphyritic constituents during metamorphism. The new porphyritic 

 minerals may show random orientation if formed under mass-static condi- 

 tions, or parallel orientation if formed under conditions of mashing. The 

 augen-gneisses are cases of this kind. For instance, the augen of one 

 gneiss may be the partly mashed original porphyritic feldspars of an 

 igneous rock, while those of another gneiss may be newly developed 

 porphyritic constituents in a metamorphosed sediment. In such cases it is 

 difficult or impossible to determine from the appearance of the rock whether 

 an augen-gneiss is of igneous or of sedimentary origin. 



(5) After igneous rocks have been deformed in the zone of fracture 

 the openings may be filled by cementation. Subsequent mashing may 

 break the fillings into fragments, and shearing may give them roundish 

 forms which very closely simulate pebbles. This happens in more instances 

 with quartz veins than with other minerals. Such metamorphosed schists 

 bear numerous apparent pebbles of quartz, and are very difficult indeed to 

 discriminate from metamorphosed sedimentary rocks which contain true 

 waterworn pebbles of quartz. 



(6) A final class of cases in which it is very difficult to determine what 

 part of a rock mass is igneous and what part is sedimentary is that in which 

 a sedimentary rock has been injected in a complicated fashion by one or 

 more igneous rocks, and later the combined mass is metamorphosed. Such 

 a schist is derived partly from a sedimentary and partly from an igneous 

 rock. Some cases are as follows: 



A metamorphosed fissile sedimentary rock, such as mica-schist or mica- 

 gneiss, may be injected in a complicated way parallel to the schistosity, and 

 thus produce a banded gneiss, part of which is igneous and part sedimentary. 

 The rock may be predominantly of either one of these materials. If the 

 injected sedimentary rock be subsequently folded, this will produce differ- 

 ential movements parallel to the banding, and the igneous and aqueous 

 bands may be merged into one another and have structures sit similar that 

 it is impossible to determine what part of the rock is igneous and what part 



