PETROORAPHIC GEOLOGY AND DESCRIPTIONS. 193 



Aporhyolitc.] 



Angle 001 A 010= 94 40' (calculated by Descloizeaux, at 93 200. 



Angle 130 A 010=145 10 '. 



Angle 110 A 010--118 30' (?) (calculated by Descloizeaux, at 120 53 '). 



Angle 001 A OTO 83 15 ' (observed by Marignac, 86 40'). 



Angle 021 A 010 -133. 



Angle TIT A 010-114 50' (observed by Marignac, 114 40'). 



Angle 02T A 001 135 20'. 



1 hina gives none of the exterior angles of labradorite. Descloizeaux has men- 

 tioned those measured by Marignac, and has calculated several others.* 



Of the angles measured from this crystal, the following are not given by 

 Marignac, as quoted by Descloizeaux: 02T A 010; 02T A 001; 130 A 010. 



The crystal has a gray color, but when broken it is very pure and glassy. Thin 

 sections cut at random show that it is sparsely twinned on the albite plan, and appa- 

 rently on the pericline plan. 



Chemical analysis (No. 250) by Prof. C. F. Sidener, gave the following results: 



Silica, SiO 2 50.75 per cent. 



Alumina, A1 2 O 3 32.80 



Ferric oxide, Fe 2 O, .22 " 



Ferrous oxide, FeO none 



Calcium oxide, CaO 13.69 



Magnesium oxide, MgO .04 " 



Potassium oxide, K 2 O .12 " 



Sodium oxide, Na 2 O 2.60 " 



Titanic oxide, TiO, none 



100.22 



Calculated as a plagioclase, this composition indicates a feldspar between 

 AbiAn., and Ab,An 3 , or labradorite-bytownite. 



Five sections and three preparations. 



Age. Cabotian. 



Remark. In several places crystals of labradorite have been seen in the great 

 gabbro mass of the state, and sometimes even larger than this, but they were firmly 

 embraced in the matrix. Such have been seen frequently to have the size ofan 

 inch or more across the brachypinacoid and rarely five or six inches. But the 

 crystal here described is larger than any hitherto extracted from the rock elsewhere in 

 Minnesota, and we have not been able to find a description of a labradorite crystal 

 so large described from any place. N. H. w. 



No. 129. APORHYOLYTE. 



An isolated outcrop a short distance northeast of the mouth of Beaver creek, in Beaver bay. Lies on 

 No. 130. 



Compare No. 140. Forms a buttress fifty-five feet wide and twenty-five feet high. Apparently dips south 

 at 30. 



Ref. American Association for the Advancement of Science, vol. xxx, page 161; Annual Report, ix, pages 

 21, 33; Annual Report, x, page 141. 



* Jfanue? <Ze Mini >-(t!n<iir, Toiuei, p. 303. 

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