TEXTURE AND VARIETIES OF GNEISS. 371 



mica may be either potash mica or magnesia mica, and occasionally 

 both micas are found in the same rock. Sometimes the mica sur- 

 rounds the crystals of felspar, giving that mineral a lenticular form. 

 Hornblende is an important constituent in many gneisses of the West 

 of Scotland, and chlorite and talc are found in some gneisses of Scot- 

 land, so that gneiss has often been divided into mica gneiss, hornblende 

 gneiss, and chlorite gneiss. 



Texture of Gneiss. Gneiss varies in texture with the condition 

 of the mica. In the common type, mica is found in separate laminae, 

 dividing the felspar and quartz. But when the foliated texture is 

 indistinct, owing to the imperfect continuity of the mica films, the rock 

 is termed granitic gneiss, and makes a transition to granite. On the 

 other hand, the mica may be so abundant as to isolate the quartz and 

 felspar in lenticular masses ; and in section this condition gives a 

 delicate veined aspect to the rock. Sometimes the mica shows 

 parallelism, giving the foliye of the rock as regular an aspect as 

 exogenous growth in wood ; and this condition further developed 

 imparts a platy cleavage to the gneiss. 



Though the disintegrated materials of granite sometimes com- 

 pose the substance of gneiss, fragments of granite are most rarely 

 discovered in it. 1 But while gneiss may sometimes be a product 

 of denudation of granite subsequently metamorphosed, more frequently 

 the gneiss is simultaneously formed ; and granite is only the same 

 rock material more irregularly crystallised. 



Transition from Gneiss to Granite. Professor Heddle recognises 

 three varieties in this form of metamorphism. First, there is a gradual 

 increase of granitic texture over the gneissic texture. This condition 

 may be seen on the north-east of the hill of Scoltie, near Banchory, 

 where granitic veins of segregation or veinstones appear in the gneiss, 

 while a couple of miles north the gneiss is becoming granite, owing 

 to a steady increase in the number of the granitic bands. The veins 

 themselves become larger, are free from foliated texture, and the granite 

 was a fine grain. Further on towards the hill of Fare the granite bands 

 progressively augment, and the gneissic bands are seen dwindling 

 away. 



The second form of granitic metamorphism is abrupt, and is well 

 seen in the quarries of Tillyfourie, where the gneiss is highly con- 

 torted, broadly banded, and passes suddenly into granite without any 

 trace of intermediate mineral condition. Yet there is no space where 

 the one can be said to be in contact with the other. The openings 

 between the folias of the gneiss cease to exist along a wavy course, the 

 only sign of change being that sometimes the dark mica of the granite 

 has its plates parallel to the line of the transition near to the un- 

 altered rock. 



Thirdly, there is metamorphism by a gradual fading of granite into 



1 MacCulloch says that in certain varieties of mica schist fragments of granite, 

 of quartz rock, and of limestone are embedded in it. John Phillips suggests 

 that he may not always have been careful to avoid admitting conglomerates among 

 mica schist and gneiss. 



