GEOLOGY OF THE LONG LAKE QUADRANGLE 47 1 



was comparatively even, which is far from likely. Near the present 

 borders of the mass inclusions of the older rocks are found, suggest- 

 ing that here we are near the actual upper limit. Since the rock 

 was formed the mass has been much dislocated by faulting, shifting 

 the relative levels of the old surface in the various fault blocks. 

 This also urges caution in assuming that 3000 feet or more have 

 been generally worn away from the valley regions, and also ren- 

 ders it certain that quite different amounts have disappeared 

 from the surfaces of the various fault blocks. If however the 

 present surface were not far beneath the original surface it would 

 seem that inclusions of older rocks should be more common than 

 they are in most of the anorthosite district, and that we should 

 also find downfaulted blocks of other rocks within it. So far as 

 the writer is aware, such phenomena are mainly confined to the 

 borders, and thus a quite respectable amount of wear from the 

 upper surface is argued. 



The rock solidified as an exceedingly coarse porphyry, large 

 crystals of labradorite feldspar, often several inches in length, 

 abounding, surrounded by smaller crystals of the same material, 

 for as a whole the rock is made up of this mineral, other constitu- 

 ents being present only in very minor degree. The large crystals 

 are of deep, blue-black color, often iridescent, and show bright, 

 glistening cleavage faces, on which twinning striations are usually 

 plainly observable. Originally the remainder of the rock was of 

 the same color and in the least metamorphosed portions, when 

 unweathered, it is today. 



Changes of composition are observable, both locally within 

 the mass, and quite uniformly as its border is neared. These con- 

 sist in increase in amount of the other rock constituents, with 

 corresponding diminution of the feldspar. This may continue 

 until they equal the feldspar in amount, and in exceptional cases 

 exceed it, but these are extreme phases and one rock slowly grades 

 into the other. These other minerals are augite and titaniferous 

 magnetite, which are present everywhere in the rock in small 

 quantity, and hornblende, hypersthene and garnet, which are 

 not everywhere present, but are universal in the less feldspathic 

 portions. Chalcopyrite is a widespread constituent, though in 

 small quantity. The garnet commonly forms zonally around the 

 magnetite, separating it from the feldspar, and the black center 

 with the red zone of garnet surrounding it is a very common 



