REPORT OF THE DIRECTOR igi2 101 



anorthosite, or have been folded into the latter and metamor- 

 phosed.'' 



From these statements we see that three possible modes of origin 

 of these garnet-bearing beds have been suggested, namely that they 

 are: lenses of sedimentary rock actually included in the igneous 

 rock ; or sediments folded into the igneous rock and metamor- 

 phosed ; or sheets or dikes of very basic igneous rock. Now the 

 work of the writer shows that, without question, these garnets 

 occur in lenses of Grenville sediments 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 (in- 

 clusions) of sediment which were pretty low in silica. These in- 

 clusions are portions of a great thickness of hornblende-garnet 

 gneiss, frequently interbedded with limestone, of the Grenville 

 series. This gneiss is a basic rock generally carrying several per 

 cent of magnetite ; sometimes considerable hypersthene ; and little or 

 no quartz. It is quite likely that some of the closely involved lime- 

 stone was mixed with the inclusions of sediment during the process 

 of metamorphism. It will at once be seen that such an iron-rich, 

 silica-poor sediment w^as very favorable for the development of 

 large garnets under the conditions of great heat and pressure which 

 were brought to bear upon the lenslike inclusions in the molten 

 syenite or granite. 



The hornblende rims or envelops are quite certainly great reaction 

 rims around the garnets, but just at what stage of the metamorphism 

 they were produced is not at all clear to the writer. The rounded 

 character of the garnets shows pretty clearly that the rims of horn- 

 blende are of secondary origin and that they were formed some- 

 time after the crystallization of the garnets and possibly at the 

 time when the pressure producing the foliation of the rocks of the 

 region was brought to bear. 



In case number 4 (Hooper mine) a clew to the origin of the 

 garnets is furnished by a study of the wall rock in the Rogers 

 mine on Gore mountain. In this latter case the typical garnet- 

 bearing rock (No. I of the accompanying table) of the mine passes 

 by perfect gradation, through an eight or ten foot zone, into 

 a basic syenite or acidic diorite (No. 2 of the table) which contains 

 distinct dodecahedral garnet crystals up to over an inch across but 

 always without hornblende rims. This rock, in turn, grades into 

 a hornblende (quartzless) syenite (No. 3 of the table) which merges 

 into the typical country rock of quartz, hornblende syenite, these 



