Vol. 60.J METAMORPHISM IN THE LOCH-LOMOND DISTRICT. 



25 



ALBITE- 

 GNEISS 



1 



Inverugl 



ea.ij :u1 Ardain 

 .. / 



Inversnaid 



MICA- 



which grits predominate, almost every rock being- a fine or a coarse grit ; 

 then the zone of mica-schists, in which a few of the coarser grit-bands 

 still survive owing to their resistance to dynamic metamorphism ; 

 then the zone of mica-schists, composed entirely of authigenic minerals : 

 and finally the zone of albite-gneiss. To the south-west, in Cowal, 

 the boundaries of these zones cross, a circumstance which makes 



the study of the meta- 



Fig. 3.— Sketch-map of the Loch-Lomond morphism much more 



district, to illustrate the zones of meta- difficult, as schistose grits 



morphism. (Approximate scale: a miles ma - v be found stratified 



= 1 inch ) with albite-gneisses : the 



former representing beds 

 which have resisted suc- 

 cessfully the dynamic 

 metamorphism, and con- 

 sequently to a great 

 extent the constructive 

 metamorphism ; while the 

 latter are the beds which 

 have succumbed to the 

 dynamic, and so fallen 

 an easy prey to the con- 

 structive, metamorphism. 

 The crossing of the boun- 

 daries of these meta- 

 m orphic zones probably 

 takes place also north of 

 Glen Gyle : it is demon- 

 strative proof that the 

 albite-gneisses arc not 

 strati graphical group. 

 Loch-Lomond dis- 

 is remarkable in 

 being practically free 

 from such confusing 

 complications. 



SCHIST- 



/ GRITS 



Dubh' 



,/.i, r AND 



MICA-SCHIST^./ ' 



WITH 

 'GRIT-BANDS,,-- 



S L ATES 



a 



The 

 trict 



[The asterisks indicate localities where speci- 

 mens of the rocks described in this paper 

 were obtained.] 



YIII. Contact-Metamokphism. 



One other phase of metamorphism in the district must be recorded 

 briefly, and that is contact-metamorphism. In the albite-gneiss 

 area are numerous intrusions of what are called the 'newer granites 

 and diorites ' ; that is to say, a series of intrusions well-known 

 in the Highlands, and probably in the main of Old-Ked-Sandstone 

 age, which have been intruded after the regional metamorphism. 

 The effect of these intrusions on the albite-gneisses is remarkable. 

 On approaching such an intrusion as the Beinn-Yane (Mheadhoin) 

 diorite or the Inversnaid hyperite, the albites are seen to assume a 

 red coloration, due to the oxidation of the magnetite-inclusions, while 



