Invasion to Regional Metaniorphism, 3 



heated and softened bottoms of geosynclines. Orogenic 

 deformation was looked npon in connection with the 

 trapped sea waters as the agent whicli accomplished the 

 work. The newer interpretations, however, regard the 

 fluids and gases of igneous rocks as of juvenile origin, 

 making their way for the first time toward the surface 

 of the earth. These present views in regard to the origin 

 of the granitic rocks and their emanations raise the prob- 

 lem as to how far igneous action is essential for the 

 development of regional metamorphism. To what extent 

 does regional metamorphism imply the invasion of deep- 

 seated bodies of magma of regional extent, even when 

 these give no direct evidence of their presence; or to 

 what extent may mere crustal compression and deforma- 

 tion of deeply buried rocks carry forward this work! 



Since, by the very terms of this question, the magmatic 

 invasions, if they are the controlling causes of regional 

 metamorphism, are concealed, the problem can not be 

 directly solved. It must be attacked by inference, but 

 the attack is none the less desirable, since it raises 

 problems with many bearings, and directs the attention 

 to the observation of field relations. The accumulation 

 of these from many sources and by many minds must in 

 the end give adjusted quantitative value to the opposing 

 factors and lead to a better understanding of the 

 mysterious inner earth. 



The problem and the conclusions, as outlined in this 

 article, were formulated in the mind of the writer from 

 1904 to 1906, and presented more or less completely to 

 successive classes of advanced students. The points for 

 first presentation are profound metamorphism of some 

 large areas in New England where igneous rocks are 

 near by, but the depth of burial slight ; and the relatively 

 small effects in a Pennsylvania region where igneous 

 rocks are absent, but the depth of burial has been very 

 great, and folding fully as close as in the New England 

 region.^ 



The fact that in all the intensely metamorphosed 



^ [The degree ol' intensity of metamorphism is judged by the usual crite- 

 ria: deformation of original structures and textures; formation of new 

 minerals; recrystallization ; shape, size, and arrangement of grains with 

 resulting cleavage and luster; and changes in the composition of the rock. 

 The changes that occur without notable additions were discussed by Lahee in 

 a paper on the crystalloblastic order and mineral development in metamor- 

 phism (Jour. Geology, 22, 500, 1914), but in this paper instances are cited 

 where the changes are more profound and result from magmatic additions.] 



