442 FRANK F. GROUT 
A classic example of banding is that in the gabbro mass of the 
Isle of Skye (1, 2).! There are many parallel layers of lighter and 
darker material, and some of the bands curve conspicuously. 
Another prominent case is that on Orn6é (3) just south of Stock- 
holm, where the alternate bands are black and white, and the 
banded rock is said to constitute the periphery of an intrusion. An 
equally notable color banding appears in the large igneous Ilimausak 
rock in Greenland (4). The bands are from one to three meters 
thick, and three main rock types alternate with remarkable regu- 
larity. ‘The bands are saucer shaped in a large way and there are 
no apophyses between bands. ‘Transition zones are narrow and 
the texture is unchanged at the contacts. 
The Laurentian gneisses have a banding that is in some places 
clearly an original igneous structure (5). Some bands pinch 
out, and all are notably different from the roof in composition. 
There are no sharp contacts and no transverse dikes, though some 
related pegmatites cut across the bands. Many papers on Canadian 
igneous tocks mention structures of this sort (6, 7,8). The banded 
rocks studied under the microscope show in the most positive 
manner that the structure developed while the rock was still 
molten, or at most only partly crystalline. There are in many 
specimens no traces of mineral deformation; nor is there any 
reason to suppose that recrystallization has obscured the signs of 
some previous deformation. Mount Johnson, near Montreal (9), 
shows bands rich in feldspathic material alternating with others 
richer in iron and magnesian constituents. The dip and strike 
can be measured. The alkali syenites of eastern Ontario (10) 
show bands. The Sudbury norite is reported by Mr.. Hugh 
Roberts, of Minneapolis, on the basis of recent exploration, to show 
an alternation of mineralogically differing bands. 
The Cortlandt series in New York has an “original gneissoid”’ 
structure in which the bands differ in mineral composition. While 
there are sharp contacts, it is characteristic that the grains in all 
cases interlock across the contact. None of the series exhibits any 
great amount of shearing (11). In the Adirondacks, bands one to 
one hundred feet thick show alternating gray and pink colors (12). 
1 Numbers refer to entries in the bibliography at the end of the paper. 
