488 J. J. jS. Teall — Origin of Banded Gneisses. 



present for the most part in irregular grains. Sometimes, however, 

 we find more or less rounded grains of quartz inclosed in the 

 felspar, so that the quartz was not always the last mineral to form. 

 The mica is deeply coloured. It occurs in detached scales and also 

 in aggregates composed of several individuals. Sections parallel to 

 the vertical axis appear a pale yellowish-brown when viewed with 

 rays vibrating at right angles to the cleavage cracks, and opaque or 

 a deep reddish brown when viewed with rays vibrating parallel to 

 these cracks. The mineral is biaxial with a very small optic axial 

 angle. It evidently contains a considerable amount of iron, for the 

 rook in which it is abundant often assumes a deep red colour by 

 weathering. The mica of the weathered rocks has been replaced by 

 chlorite with which iron-oxides are associated. 



The more modified rocks, and these are by far the most abundant, 

 are characterized by the presence of a fine-grained granulitic aggre- 

 gate of quartz and felspar and a more or less marked parallel 

 structure. The granulation of the quartz and felspar is, I believe, 

 a secondary phenomenon due to the dynamic metamorphism which 

 has operated upon the district. It may be developed merely at the 

 margin of the original grains, in which case we have the mortar- 

 structure of Tornebohm ^ — the relics of the original grains then lie in 

 a fine-grained granulitic paste like stones in mortar — or it may 

 replace entirely, or almost entirely, the original quartz and felspar. 

 The development of white mica does not appear to be a common 

 feature. It may, however, be observed in what I am inclined to 

 regard as a schistose granite from Porthalla, where it occurs along 

 the planes of schistosity. In this rock the secondary granulitic 

 aggregate forms a large portion of the mass. 



A very interesting variety of gneissose rock occurs in the serpentine 

 near Kynance. It forms only a thin band, and is, I believe, a granitic 

 vein which has been profoundly modified by the regional meta- 

 morphism. Sections at right angles to the foliation show a 

 magnificent micro-flaser structure. The secondary aggregate of 

 quartz and felspar winds in and out amongst the relics of the 

 original grains, and possesses in some places a crypto- rather than a 

 micro-crystalline structure. The rock does not possess any very 

 definite schistosity and white mica is absent. It appears as if the 

 development of white mica were connected with movement along 

 definite planes — where the mass has been deformed in a plastic 

 manner white mica is absent. 



The specific gravity of the rocks of the granitic type lies between 

 2-6 and 2-7 



Diorite. — This rock, like the granite, never occurs in large in- 

 dependent masses. It is generally veined by the granite or intimately 

 banded with it in the gneissic series. It may also be found in the 

 gabbro. The rocks to which I here apply the term diorite vary in 

 composition and specific gravity. Some belong to the intermediate, 

 others to the basic series. At the one end of the series we have a 



1 Naagra ord om granit och gneiss. Geol. Foren. i Stockholm Forhandl. 1881, 

 pp. 166 u. 233. 



