GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



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somewhat more basic than the usual rock, and this seems to be 

 true of all such syenite, so far as can be judged by the thin sec- 

 tions. Garnet is much more abundant than in the usual syenite, 

 and bronzite is lacking. The analysis indicates a rock composed 

 of 24.25$ orthoclase, 44.55$ albite, 5.6$ anorthite, 1.7ft magnetite, 

 5.8$ garnet, 14.5$ augite and 3$ quartz. If the ferrous iron be 

 too high, and this is possible, though the discrepancy can not be 

 great in this case, the magnetite and anorthite percentages 

 would be slightly increased and those of augite and quartz 

 diminished. Except for a slight amount of acid plagioclase, the 

 feldspar is all of the intergrowth types, and cleavage fragments 

 from the crushed rock show the optical characters of anortho- 

 clase, viz a + 9° extinction on M, with an acute bisectrix in the 

 center of the field. 



The rock used for the next analysis, 9, is from near the anorth- 

 osite boundary. Ferro-magnesian silicates are more prominent 

 than usual, considerable hornblende, augite and garnet being 

 present and some bronzite, altogether constituting some 30$ of 

 the rock. The lower alkalis show the diminished feldspar per- 

 centage, but a calculation is rendered impossible by failure of 

 the ferrous iron determination. On the basis of 5$ of magnetite, 

 as indicated by the separation previously mentioned, the calcula- 

 tion gives a silica residue amounting to 13$ of free quartz, which 

 is much too high, there being but little present. The remaining 

 analyses require little comment aside from no. 10. No. 11 is 

 regarded as giving the closest approximation to the mean com- 

 position of the rock and is from the Loon lake type locality. 



The pyroxenes and hornblende which these rocks contain are 

 precisely like those in the anorthosites, strongly suggesting com- 

 munity of origin. The feldspars are alkali feldspars with 

 closely corresponding soda and potash content. In the general 

 rock garnet is a much less conspicuous feature than in the 

 anorthosites, and is often wholly absent. This is but natural, 

 since the garnet is not primary but has resulted from the inter- 

 action of feldspar and magnetite. It is a lime-iron-alumina 

 garnet, and the necessary lime for its formation is lacking in the 

 alkali feldspar of the syenite. A further distinction between 

 the two rocks lies in the abundance of zircon in the syenite. It 



