2 J. Barrell — Relations of Subjacent Igneous 



Batholiths like those in the American Cordillera seem 

 to come to place without crustal compression, but those of 

 the Archean shield and those of the later Appalachian in- 

 vasions are accompanied by compression. A detailed 

 study of three or four regions shows the metamorphism to 

 be related to the igneous invasion more than to the depth 

 and pressure. One of the regions of deepest burial and 

 close folding in Pennsylvania shows slight metamor- 

 phism. 



II. The action of magmas, both by heating and meta- 

 somatism, is reviewed. The solutions are not meteoric in 

 origin. The results in minerals depend on equilibria, — 

 largely on the presence of HoO and CO2. 



The depth of anamorphism may be small, due (1) to 

 weakness of some rocks, (2) to invasion of batholiths. 

 An argument for shallow depth is based on the complete- 

 ness of Archean metamorphism and the salt of the ocean 

 as a measure of erosion. 



III. The features of metamorphic rocks are reviewed 

 and interpreted as due to one or another factor. Major 

 factors are batholithic invasion and compression. Move- 

 ments of solutions, selective crystallization, lit-par-lit in- 

 jection gneisses, and the alternation of injection and 

 mashing, each leave their marks.] 



PART I. REGIONAL RELATIONS. 

 Introduction. 



The phenomena of regional metamorphism are mani- 

 fested in a widespread crystallization and reconstruction 

 of rocks, in granulation and recrystallization. Mashing 

 and shearing compete with folding, as three distinct 

 modes of yielding, in giving complicated structures ; 

 mashing and recrystallization lead to segregation of 

 minerals in layers, and are manifested in a doniinant 

 foliated and banded arrangement which formerly was 

 taken as evidence of sedimentary origin. Metamorphosed 

 terranes pass in places into massive holocrystalline rocks 

 which are now proved in most cases to be of intrusive 

 igneous origin. In rare cases, however, they seem to be 

 completely recrystallized sediments. A former interpre- 

 tation which prevailed in this country until near the close 

 of the nineteenth century regarded all these gneisses, 

 including the granite ones, as merely the last term of 

 metamorphism of sediments in place, especially in the 



