ASSIMILATION OF THE SYENITE-GRANITE MAGMA 



257 



Similar phenomena exhibited on a much more extensive scale may be 

 seen in excellent outcrops along the creek from one-fourth to one-half 

 mile below Dunning Pond (Lake Pleasant quadrangle). The variable 

 red, granitic gneiss is so full of gray Grenville inclusions as to make up a 

 considerable percentage of the rock. There are all stages, from thor- 

 oughly fused in or assimilated Grenville to some which has been but little 

 affected. 



■ A very definite case of the assimilation of the border of a large Gren- 

 ville gneiss inclusion by the inclosing syenite is shown at the Eogers gar- 

 net mine^* on Gore Mountain (Thirteenth Lake quadrangle). This in- 

 clusion, which has a length of three-fourths of a mile and a maximum 

 width of 100 feet, consists of a medium grained hornblende-garnet 

 gneiss. This typical garnet-bearing rock (number 1 of the table below) 

 passes by perfect gradation through an 8-foot or 10-foot zone into a basic 

 syenite or acidic diorite (number 2 of the table), which contains distinct 

 dodecahedral garnet crystals up to an inch or more across. This rock, in 

 turn, grades into a hornblende (quartzless) syenite (number 3 of the 

 table) which merges into the t3rpical country rock of quartz-hornblende 

 syenite, these two latter rocks being at times somewhat garnetiferous. 

 This transition zone has certainly been formed by assimilation or actual 

 fusing together of the syenite and the border of the great inclusion at the 

 time of the intrusion. 





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At the Hooper garnet mine, just east of the northern end of Thir- 

 teenth Lake, the whole mass of rock mined is almost precisely like the 

 transition rock just described. All evidence points to the origin of this 

 Hooper mine rock as due to pretty thorough melting of an admixture of 

 syenite and Grenville sediment where the Grenville inclusion was perhaps 

 deeper down in the magma, or possibly a number of smaller hornblende 

 gneiss inclusions, maybe with some limestone, were assimilated by the 

 molten syenite. 



55* W. J. MiUer : Econ. Geol., vol. 7, No. 5, 1912, p. 500. 



