22 MB. J. A. DOUGLAS ON GEOLOGICAL SECTIONS [vol. lxXVl r 



distinguish it from the quartz with which it is associated. Its refractive 

 index is higher than that of the original andesine, and it would therefore 

 appear to be a more basic variety, the addition of lime being possibly 

 explained by the disappearance of the numerous inclusions of hornblende. 

 The change is accompanied by a gradual loss of twin-lamellation. 



(A 83) Metamorphosed granodiorite. (Kil. 149.) A specimen inter- 

 mediate in character between the abnormal acid granodiorite and the 

 true banded gneiss into which it passes. The effects of dynamic action 

 have now become well marked. 



The rock has a peculiar and characteristic appearance in a hand-speci- 

 men, due to the development of clusters of biotite. Much of the felspar 

 is of a dull greenish colour, suggestive of saussuritization. 

 Microscopic characters : — Hornblende has now entirely disappeared, and 

 the chloritoid patches have been converted into aggregates of clear biotite- 

 flakes, most of the magnetite having been absorbed in the process. 

 Occasional clusters of magnetite-grains associated with epidote, however, 

 are also visible. 



Although a certain amount of andesine is still present, the bulk of the 

 triclinic felspar appears to be albite, having a refractive index distinctly 

 lower than that of quartz and balsam. That this change may be regarded 

 as one of saussuritization is shown by the numerous aggregates of 

 zoisite-grains scattered throughout the felspar substance. With these 

 are associated larger grains of epidote and a green mineral, showing pleo- 

 chroism and a moderately-high double refraction, which appears to be 

 hornblende. In addition to these inclusions, the alkaline felspars are 

 crowded with flakes of colourless mica, a type of alteration which seems 

 to belong to a comparatively late stage in the dynamic metamorphism. 



Peripheral granulation of the felspar-crystals, which ultimately leads 

 to the formation of an albite-mosaic, is also visible, and the orthoclase 

 shows indications of microcline structure. 



(A 81) Gneiss. (Kil. 145.) This specimen represents the most advanced 

 stage in the dynamic metamorphism of the granodiorite, being the final 

 product in the transition from A 83. 



It is a uniform pink banded rock of splintery fracture and extremely 

 fine grain. The ferromagnesian minerals are no longer recognizable, each 

 individual crystal or group of crystals having been drawn out into 

 microscopic thread-like lines. 

 Microscopic characters : — The rock exhibits pronounced cataclastic struc- 

 ture. The coarse quartz-mosaic has become crushed or drawn out in the 

 direction of movement, individual grains having been fractured, ground 

 down, and rewelded into a finer mosaic, together with a mass of clear 

 recrystallized felspar. 



A few of the larger crystals of felspar having partly escaped this 

 grinding process, except for marginal granulation, remain embedded in 

 the granular matrix (mortar- structure), and show distortion and fracture 

 of the twin-lamelhe. 



Hornblende is entirely absent, and the scaly aggregates of biotite 

 have become elongated into flattened lenticles or long drawn-out lines. 



A few crystals of apatite still persist ; but epidote and magnetite are 

 not abundant. 



(A 79) Basic gneiss. (Kil. 139.) (PL II, fig. 3.) This specimen shows 

 the effect of intense dynamic action on the more basic type of grano- 

 diorite rich in xenoliths. 

 Microscopic characters : — The constituent minerals have a strongly- 

 marked parallel arrangement. Biotite and hornblende are present in 

 about equal quantity, and tend to occur in alternating bands — due to the 

 breaking-up and drawing-out of individual crystals. 



The original crystals of hornblende have broken along their cleavage,. 



