390 THE CRYSTAL FALLS IROl^-BEARmG DISTRICT. 



nected is not known, but the possibility must be admitted. The banded 

 gneisses are often so faintly foliated and resemble the granites so closely in 

 color and grain that the distinction can be made with only the microscope, 

 and igneous contacts between them might easily be overlooked. It is 

 certain, however, that if the granites have been injected into the banded 

 gneisses it has not been in the form of narrow dikes, and the fact remains 

 that no case of an igneous contact is recorded in our notes. 



The gneissic members of this division are merely crushed granites, and 

 owe their foliation partly to the crushing and partly to the growth of fresh 

 mica in the fractured zones. They differ from the banded gneisses in fur- 

 nishing both field and microscopic proof of the way in which the foliation 

 was formed and of the rocks from which they were derived. 



(2) The banded gneisses of the second group have essentially the same 

 mineral composition as the granitic gneisses of the first. They are distin- 

 guished by the eye mainly by the fact that the component minerals occur in 

 more or less distinct layers, from a fraction of an inch upward in thickness. 

 The lamination, which only rarely is very regular, seems to be caused in most 

 if not in all cases by the alternation of darker layers, which are relatively 

 rich in biotite, with lighter layers, which are comparatively and sometimes 

 wholly free from it. The light layers are almost always coai-ser in texture 

 than the darker, and frequently are coarsely pegmatitic. The individual bands 

 are not indefinitely persistent, but wedge out to knife-edges. The banding is 

 sometimes so indefinite as to be lost in the hand specimen, the large surface of 

 an outcrop being necessary to bring out the slight differences in shade. In 

 color these rocks are light gray, dull -white, or pink. The banding shows great 

 variations in angle of dip, but the strike is usually fairly constant within a few 

 degrees of east and west. In a few localities distinct contortion was observed 

 in the gneissic banding and pitching folds. The lamination of these gneisses 

 is; so far as observed, of the plane-parallel type. The bauds are thoroughly 

 welded together, and as a rule, the rock breaks indifferently across them. 



Under the microscope the composition of these rocks does not differ 

 from that of the granitic rocks of the first division. The structural charac- 

 ters, however, are in strong contrast. Even in those specimens which 

 possess the most indistinct foliation all the minerals are elongated in a 

 common direction. While the individual grains in most cases show more 

 or less strain and are frequently fractured, their mutual boundaries are 

 usually sharp and clear, and it is evident that the forms are not the direct 



