CRYSTALLINE BASEMENT 47 
“tural planes of the schist mass. Crosscutting dikes are occassionally 
“seen, but more irregular bodies with sinuous contacts and numerous 
_apophyses are not met with. Brecciation, melting and assimilation 
are also odd phenomena, except a certain class of banding, 
‘where a lateral assimilation on a very reduced scale has suc- 
ceeded. The whole appearance of the intrusion depended appa- 
rently upon a lateral controlling pressure, causing the mentioned 
parallel injections along the planes of weakness. The areal exten- 
sion of the resulting banded rocks is a very considerable one. Alt- 
hough they are of the same origin as many common parallel ar- 
terites in archaean terranes, the banding in the Umango area shows 
‘a somewhat different character, due to an already mentioned lateral 
assimilation along the granite veins. The lateral assimilation is com- 
mon especially where the granite intrudes amphibolite schists, from 
which results a product of intermediary composition. This last is 
also banded, evidently a result of parallel movements under stress 
conditions. Also at the borders of other schists such a banding 
was produced. 
The banding phenomenon has been studied already long time ago 
in different archaean terranes. The following two classes may be 
distinguished: 1) Banding produced by flowing in a primary magma 
with the formation of „Schlieren“ or streaks of different composition. 
The well known banded gabbro of Skye, Scotland, described by 
Geikie and Teall! belongs to this type. 2) Banding produced by 
the injection of magma along the structural planes of a schist mass. 
This type is the more important one and is of regional extension, 
as is shown by investigations in many parts of the world.? 
oo 1A. Geikie and J. J. H. Teall: Banded Igneous Rocks of Skye. Quarterly 
Journal of the Geological Society of London. Vol. L. 1894. 
2J. J. Sederholm: Om granit och gneis. Bull. Comm. Géol. de Finlande. 
N:o 23. Helsingfors 1907. 
P. J. Holmquist: Zur Petrographie und Geologie von Ornö Hufvud. Bull. 
Geol. Inst. of Upsala. Vol. X. 1910. 
Adams and Barlow: Geol. of the Haliburton and Bancroft Areas, pro- 
