666 THE GEOLOGY OP MINNESOTA. 



[Esterellyte. Sycnyto. 



Mir. This rock consists very largely of secondary quartz in interlocking 

 growths which surround the old feldspars, which are much twinned and also much 

 altered. In this interlocking growth are also irregular, apparently fragmental 

 masses of aiujite and a few of spliene, as well as a few spicules of green hornblende. 

 The feldspars are strikingly zoned, and are intergrown, apparently along the easy 

 cleavages, in the manner of a microperthite, by another feldspar of later date, this 

 later growth being somewhat interlocked about the margins in the secondary quartz. 

 In this rock some of the small augite crystals are entirely included in the old 

 feldspars. The same is true of some of the sphenes. One section. 



Age. Archean (Keewatin). N. H. w. 



Remark. This rock is closely similar to No. 551U, which is a part of this same 

 granite mass. No. 551 G is from the south shore of Kekequabic lake in S. W. \ S. 

 W. i sec. 3, T. 64-7 W. 



An analysis of the whole rock is as follows: 



SiO 2 66.84 



A1 2 O 3 18.22 



Fe 2 O, 2.27 



FeO 0.20 



CaO 3.31 



MgO 0.81 



K 2 O 2.80 



Na 2 O 5.14 



P 2 O 6 trace 



H,O 0.46 



Total - 100.05 



On separating the powder of a fresh specimen of the normal granite (No. 551< ! ) 

 by means of Thoulet's solution the larger proportion of the feldspar fell between a 

 specific gravity of 2.58 and 2.62, which would indicate that the mineral was a mixture 

 of the orthoclase and albite molecules; and the analysis, as here given, shows that it 

 belongs to the anorthoclase series: 



Analysis of feldspar. 



SiO 2 A1 2 O 3 Fe 2 O 3 CaO MgO K 2 O Na 2 O H 2 O Total 



67.99 19.27 0.82 0.75 0.02 3.05 6.23 0.90 99.03 



It is to be noticed that the silica percentage is larger than is required by the 

 amount of soda, potash and lime present. This is probably due to the fact that a 

 small amount of quartz was so intimately intergrown with the feldspar that certain 

 grains of the feldspar powder contained some quartz. From the analysis it is cal- 

 culated that this feldspar is an nnorthodase with approximately the composition 

 Or 5 Ab, 4 An,. u. s. G. 



No. 1106. SYENYTE. (Aurjitic, Esterellyte. ) 



From the same little island as the last, north side. 



Ref. Annual Report, xv, pages 3G3, 398. Compare No. 571G. 



Meg. Porphyritic, similar to the last. 



