J. Barrell — Subjacent Igneous Invasion. 255 



Art. XVI. — Relations of Subjacent Igneous Invasion to 

 Regional MetamorpMsm; by Joseph Barrell. 



(Continued from page 186.) 



PART III. INTERPRETATION OF DYNAMO-METAMOR- 



PHIC FEATURES IN THE ROOFS OF BATH- 



OLITHS IN MOUNTAIN PROVINCES. 



Preliminary Statement. 



This article has reached the point where it appears that 

 batholithic invasion is to be looked upon as one of two 

 major factors in the control of the phenomena of dynamo- 

 metamorphism, the other being crustal deformation. 

 The following part mil consequently be a study of 

 _ significant features and their interpretation as phenom- 

 ena of magmatic injection, chemical alteration, and 

 lateral compression of batholithic roofs. 



Features Produced by Movements of Solutions and Selective 

 Crystallization. 



Where blocks of biotite granite gneiss are inclosed in a 

 coarse granite, it may frequently be observed that a con- 

 centration of biotite from the surrounding magma has 

 taken place within and around such fragments. Pegma- 

 titic seams in banded gneisses usually show no change at 

 the margins, but may sometimes be seen to have a biotite 

 lining which slightly permeates the walls and gives 

 sharper definition to the seams. This tendency for bio- 

 tite to separate by selective crystallization from highly 

 aqueous solutions of magma is to be related with that nor- 

 mal sequence of crystallization in magmas whereby the 

 black bisilicates, associated with smaller amounts of other 

 minerals, crystallize out early, followed by the crystalliza- 

 tion of plagioclases, then orthoclase, then orthoclase and 

 quartz ; each phase overlapping the adjacent ones. Now 

 if the magma is rising through the foliated structure of a 

 roof and especially if the amount of water is large, so that 

 the crystals of any one generation are but a small part of 

 the magma, the result is a vertical separation of the 

 magma by fractional crystallization. The biotite will be 

 strained out in one place and crystallize against biotite. 

 Quartz and feldspar will rise higher and crystallize as 

 pegmatite free from biotite. Where the magma rises 

 through a foliated country rock, it should lead to a pro- 

 nounced banding in composition, giving banded gneisses. 



