OS NEW YORK STATE MUSEUM 



coarse grain but not of the leaf type, and holds a multitude of 

 inclusions of the other minerals, along with a little apatite and 

 many zircons. Except for these inclusions the quartz foliae are 

 entirely of that mineral. 



Between is a mosaic of garnet, augite, bronzite and feldspar, 

 with many minute graphite scales. Oligoclase is the prevailing 

 feldspar, though with anorthoclase also. Notwithstanding the 

 high quartz content, the rock holds more lime and magnesia than 

 any of those already mentioned and would seem to have been a 

 calcareous sandstone. It grades into less quartzose varieties. 



Associated igneous gneisses. Mingled with the Grenville sedi- 

 ments, often intricately, are other gneisses, some of doubtful, and 

 some of pretty distinctly igneous character. These rocks are 

 always thoroughly gneissoid, retaining no more trace of their 

 original texture than' is the case with the old sediments, so that 

 again the argument for their origin is mainly based on their 

 composition. The dubious rocks are of so many sorts and shades 

 that it is difficult to treat of them except in a mass of details 

 which would be out of place here. Some of them may likely be 

 foliated contact rocks, and others may be due to a development 

 of mixed rocks along the contacts of the aqueous and igneous 

 rocks by an interchange of materials during metamorphism, 

 though it is not at all certain that such transference ever takes 

 place to any important extent, even during very deep seated 

 metamorphism. 



The probable igneous rocks show a range from the most acid 

 granites through syenitic rocks to heavy, black rocks of gabbroic 

 composition. 



The granitic gneisses are usually of red color and mainly com- 

 posed of quartz and feldspar, the gneissoid character being de- 

 pendent on the development of quartz of the leaf type in thin 

 foliae, separated by fine quartz feldspar mosaic. The quartz 

 makes from one quarter to one half of the rock, the feldspar is 

 mainly anorthoclase or microperthite, though with some oligoclase 

 always, and sometimes a little miorocline, and the other con- 

 stituents are small amounts of zircon, apatite, magnetite and 



