20 ME. J. A. DOL'GLAS ON GEOLOGICAL SECTIONS [vol. lxxvi r 



Moreover, except in the case of the gneiss formed by the crushing 

 of the granodiorite where it was rich in basic xenoliths, in which 

 hornblende was originally present in great abundance, this mineral 

 is not characteristic of the banded rocks, and in specimens showing 

 the most advanced stage of deformation it has almost completely 

 disappeared. 



In any process of orogenic movement the rocks involved must 

 necessarily be subjected to tension as well as to compression, but in 

 many descriptions of dynamic metamorphism it is not unusual to> 

 find that too much importance is attached to the latter factor. 



It is conceivable, however, that the banding of a gneiss might in 

 some cases be due to stretching alone, and indeed a study of the- 

 various stages of the dynamic action exhibited in the present suite 

 of rocks suggests that tension has played a very prominent part in 

 the production of the gneissic structure. For example, the drawing- 

 out of more or less circular patches of biotite-flakes, first into 

 lenticles and finally into long thread-like lines, is more readily ex- 

 plained by such a process than by one of squeezing or rolling-out 

 due to compression : for, in a flexible mineral of this nature, the- 

 effects of the latter would be rendered obvious (as is not the case 

 here) by bending and distortion of the lamellae. 



That the biotite may have been in part formed pari passu with 

 the movement has already been suggested, but that its origin dates- 

 from a comparatively early stage in the metamorphism is clear from 

 the evidence cited above. 



The effects of dynamic action are indicated in the quartz at an 

 earlier stage than in the felspar. First, optical disturbances make- 

 their appearance in the form of strain -shadows ; a coarse mosaic is- 

 then developed by granulation ; and finally by increased pressure,, 

 together with solution and concomitant crystallization, this be- 

 comes broken up and converted into a mosaic of extremely fine 

 texture, composed of minute interlocking grains. Comparatively- 

 large patches of clear recrystallized quartz are often seen to have 

 been formed under the lee of a felspar-crystal which has protected 

 them from further granulation. 



Mechanical action is clearly shown in the case of the felspars by 

 the bending and fracture of the twin-lamellae, by the production 

 of secondary lamellae in the plagioclase, and microcline structure in 

 the orthoclase. The final product is a felspar-mosaic produced by 

 granulation. 



That the force has been in many cases one of tension is indi- 

 cated by the separation of fragments produced by fracture, the- 

 intervening spaces becoming filled with secondary quartz and felspar. 

 In the case of compression, overthrusting of the fracture is 

 frequently noticeable. 



Apart from purely mechanical effects, the djmamic stresses have 

 lent increased power to the agency of solution, and in this way 

 have probably been largely instrumental in causing the recrystal- 

 lization of the plagioclase in the form of albite, by the removal of 

 lime as calcite or epidote : for, where the change appeared to be due 



