174 J. Barrell— Relations of Subjacent Igneous 



^j>T. ^1.— Relation of Subjacent Igneous Invasion to 

 Regional MetamorpJiism; by Joseph Barrell. 



(Continued from page 19) 



PART II. METAMORPHIC AND METASOMATIC RELA- 

 TIONS OP OROGENIC BATHOLITHS. 



Heating and Crystallizing Effects of Magmas. 

 If subjacent magmas are a primary factor for the pro- 

 duction of regional metamorphism, they must act through 

 their heat and emanations. Their magmas, on their 

 initial rise in undifferentiated form, appear to possess a 

 heat of not less than 1200° C, but the final crystalhza- 

 tion of granite takes place at temperatures below 

 870° C. and above 575° C. This degree of cooling must 

 occupy a long time, and as the bodies of molten rock are 

 vast, there is sufficient heat passing out to raise the cover 

 rocks to a highly abnormal temperature. The curves of 

 temperature and their relation to time, as based on the 

 laws of conduction, can not give definite information, since 

 the heat transfer into the cover rocks is doubtless largely 

 by gaseous transfer, in advance of conduction.^^ Gaseous 

 transfer could, if sufficiently rapid and prolonged, heat 

 the entire cover to that thermal curve dependent upon 

 the adiabatic expansion and reactions of the gases. Dif- 

 ferentiation, by which granites are derived as a final 

 stage from antecedent more basic magmas, implies a low 

 viscosity in all of the earlier stages of magmatic rise. 

 It seems probable that there is saturation with gas under 

 pressures of 1000 atmospheres or more, and at tempera- 

 tures of the initial intrusions. The great portion of the 

 cover can then be heated up to temperatures at which 

 garnet, staurolite, sillimanite, feldspar, muscovite, and 

 hornblende can crystallize. The temperature conditions 

 for anamorphism may be brought comparatively near 

 to the surface. Above this will be a zone of hydrothermal 

 alteration. 



Chemical or Metasomatic Effects of Emanations. 

 The study of metamorphic formations shows that ex- 

 cept for the elimination of volatile constituents the bulk 



" rThe mechanism of this advance transfer of gaseous solutions has been 

 outlmed by C. N. Tenner, Jour. Geology, 22, 594 and 694, 1914.] 



