METAMORPHOSED SEDIMENTARY AND IGNEOUS ROCKS. 909 



reciystallization lias taken place. A very good illustration of this is the 

 ready recognition of the aporhyolites as volcanic rocks by the presence of 

 spherulitic and other igneous textures. Further, it has been pointed out 

 that where the conditions are mass-mechanical for rocks in the zone of frac- 

 ture, they are merely broken up into blocks or slices and that the textures 

 in the integral masses are preserved. Therefore, the origin of rocks altered 

 under mass-mechanical conditions in the zone of fracture may usually be 

 determined. Stated in another way, one may say that when the alterations 

 are of a kind to which the prefix apo is applicable, under the significance 

 given to it on pages 776-777, it is easy to discriminate between altered 

 igneous and altered sedimentary rocks. 



In contrast with the above, it has been explained that where the rocks 

 are mashed in the zone of anamorphism previous textures and structures 

 are rapidly destroyed. This may go so far as to obliterate not only the 

 textures and minor structures, but even the major structures of the sedi- 

 mentary and igneous rocks, such as bedding and differentiation. Nat- 

 urally there are all grades of this process of destruction. . It is only when 

 the mashing has gone so far as to have granulated and recrystallized the 

 rocks in the zone of anamorphism, and has thereby transformed them 

 to schists or gneisses, that original textures and structures are wholly 

 destroyed. 



CASES OF CONFUSION. 



The following are some of the classes of igneous and sedimentary 

 rocks which are most likely to be confused: 



(i) The schistose and gneissic sedimentary rocks are particularly 

 likely to be confused with the schistose and gneissic tuffs. Both classes of 

 these rocks in their original form may haA^e a lamination or banding- which 

 is roughly similar; and if the alterations go far enough to destroy the 

 textures, the rocks frequently assume a very similar aspect. 



(2) Metamorphosed sedimentary rocks derived from arkoses and gray- 

 wackes — i. e., derived from material in which the original minerals have 

 been but little sorted — are likely tp be with difficulty discriminated from 

 metamorphosed igneous rocks. This follows, first, from the fact that the 

 chemical composition of such rocks does not necessarily vary from that of 

 some igneous rocks; and, second, from the fact that arkoses and g*ray- 

 wackes are frequently very coarsely banded or even massive, so that the 



