GEOLOGY OF THE NORTH CREEK QUADRANGLE 8l 



All modes of occurrence of garnets observed by the writer on the 

 North Creek and Thirteenth lake sheets may be summarized as 

 follows: (i) as crystals or grains in various Grenville rocks, as the 

 garnet-pyroxene gneisses, the dark hornblende-garnet gneiss, the 

 gray feldspar-biotite-garnet gneisses or schists, and the white or 

 very light gray feldspar gneisses; (2) as distinct crystals fre- 

 quently occurring in all types of intrusive rocks — syenite, granite, 

 granite porphyry, and gabbro — except the diabase; (3) as large 

 more or less rounded masses, with distinct hornblende rims, in the 

 long, lenslike inclusions of Grenville hornblende gneiss in syenite 

 or granite ; and (4) as more or less distinct crystals, without horn- 

 blende rims, in a certain special basic syenitelike or acidic diorite- 

 like rock. 



In case no. i (for example Sanders Brothers and Parker mines) 

 the garnets have, in the usual way, crystallized out of masses of 

 sediments under conditions of thermal and dynamic metamorphism. 



In case no. 2 the garnets appear mostly to have crystallized 

 out of the original magmas, their presence possibly being due to 

 some assimilation of Grenville sediments, though this is by no 

 means proved. Sometimes, as in the gabbros, the garnets may 

 have been produced secondarily after the cooling of the igneous 

 masses. 



Case no. 3 (for example. Oven mountain, Rexford, and Rogers 

 mines) is of particular interest because of the very large garnets 

 surrounded by reaction rims. Without question the garnets occur 

 in lenses of Grenville sediment which were caught up or included 

 in the great igneous masses at the time of their intrusion, the 

 tremendous heat and pressure being especially favorable for a 

 very complete rearrangement and crystallization of the masses of 

 sediment which were rather low in silica. The hornblende rims 

 or envelops are quite certainly great reaction rims around the gar- 

 nets, but just at what stage of the metamorphism they v^^ere pro- 

 duced is not at all clear to the writer. The rounded and granulated 

 condition of the garnets suggest that the reaction rims of horn- 

 blende may have formed some time after the crystallization of the 

 garnets and possibly at the time when the great pressure producing 

 the foliation was brought to bear. 



A clew to the origin of the garnets in case no. 4 (for example, 

 Hooper's mine) is furnished by a study of the wall rock in the 

 mine on Gore mountain. The typical garnet-bearing rock of the 

 mine passes by perfect gradations through an 8 or 10 foot zone 



