260 J. Barrell — Relations of Subjacent Igneous 



phenomenon of lit-par-lit structure into successive intru- 

 sions of granite sheets within the country rocks, but 

 reaches typical expression within the marginal igneous 

 portion of the batholith. The injecting material changes 

 slowly by differentiation during the process from basic 

 to acid. The changing composition during the progress 

 of this action gives a conspicuously banded character to 

 the gneiss on both a large and small scale. The oldest 

 stages are dark amphibolites or diorite gneisses, the 

 youngest are fine-grained white granites. The age rela- 

 tions are definitely determinable because the younger 

 phases in places cut across the older, but more especially 

 because regional mashing is nearly always intermittent 

 with the pulsations of injection and the successive stages 

 show less and less of deformation. This is a topic of 

 sufficient importance, however, to merit treatment under 

 a separate heading. 



Ee JUVENESCENT AND DeCADENT StAGES OF INJECTION. 



The border phenomena which we study about batholiths 

 are mainly representative of the decadent stages of 

 igneous activity. An analogy with the phenomena of 

 giaciation may make this clearer. We know practically 

 nothing of the duration or oscillation of those stages 

 which preceded the culmination of the first great ice 

 sheet. Other ice sheets may have formed, advanced, and 

 retreated, yet if they did not reach the maximum limits, 

 the later glacial invasions may have obliterated their 

 record. The waning of the last continental glacier left 



origin, calling them orthogneisses ; those of sedimentary origin, para- 

 gneisses; leaving the term metagneisses for those showing definite charac- 

 teristics of neither. 



Of the orthog-neisses, the protoclastic or primary gneisses may be attrib- 

 uted to slight movement before crystals were abundant enough to greatly 

 deform each other. The orthogneisses just referred to in the text are the 

 result of successive or intermittent injection, the earlier material having 

 developed by mashing a structure that guides the later injection in parallel 

 planes. Mashing alone may develop from an igneous rock a crystalloblastic 

 orthogneiss. It is even possible that a mashed igneous rock may be injected 

 lit-par-lit or by pegmatites, yielding lit-par-lit orthog-neisses and ortho- 

 gneisses with banding of biotite and pegmatite. 



Paragneisses may be crystalloblastic gneisses, but are probably in more 

 cases developed by pegmatitic or lit-par-lit additions. 



Injection gneisses include both ortho- and paragneisses, and the injection 

 may be lit-par-lit, pegmatitic, or more normal magmatic injection. 



The terms here outlined are selected from Eosenbusch, Leith's Structural 

 Geology, Leith and Mead's Metamorphism, and Miller's paper on the classi- 

 fication of metamorphic rocks in the Bulletin of the Geological Society of 

 America, volume 28, page 451, 1917.] 



