ALMANDITE AND ITS SIGNIFICANCE 655 



Both the biotite gneiss and the biotite of the gneiss were 

 analyzed. The results of the analyses are given below: 



Biotite Biotite Gneiss 

 Per Cent Per Cent 



SiOs 36.93 55.91 



AI2O,, 22.32 18.44 



FeA 15.53 2.37 



FeO 1. 41 7.42 



CaO 2.73 3.50 



MgO 9.22 3.61 



MnO . Tr. Tr. 



K2O 3.07 1. 13 



NazO 3.12 2.08 



CO. 1 



H.0 [ 4-68 4.74 



F J 



99.01 99.20 



When the biotite gneiss is studied as a rock rather than as a 

 petrographical curiosity, it is seen to be cut by innumerable pegma- 

 tite dykes which are connected with the Laurentian granite. These 

 pegmatites have soaked the biotite-bearing rocks and it is almost 

 inconceivable but that they must have added silica, a lumina, and 

 alkalies to the original. 



THE GRANITE GNEISS 



To complete the discussion of the contact zones and the altera- 

 tion of the almandite, it is necessary to include an analysis of the 

 granite gneiss. Although this analysis is given in The Superior An- 

 alyses oj Igneous Rocks, it is not typical of the Laurentian granite 

 but rather of the granite gneiss, which is a complex or hybrid 

 rock distinguishable from the granite by a higher percentage of 

 ferro-magnesian minerals and a strong gneissic structure. The 

 granite itself contains practically no ferromagnesian minerals and 

 has at best a very poor streaked appearance when free from roof 

 pendants of gneiss. 



