662 THE GEOLOGY OP MINNESOTA. 



[Esterellyte. 



axes are shown in the accompanying figure (figure 42); the axes of elasticity of the 

 colorless variety being represented by the lighter dotted lines. 



" While in the zonal crystals there are usually only two parts, which are of 

 different optical orientation, in a few there are more than two such areas. To 

 illustrate parallel growths of this kind, the above figure is introduced. It shows 

 part of a large crystal of augite cut parallel to the cliuopinacoid. The extinction 

 and outlines of the different parts are represented rather diagrammatically, as the 

 different zones are not always separated by a sharp line. The lines a a represent the 

 parting parallel to the basal plane. The extinction angles given are those of a against 

 c. The large central part of the crystal is colorless and the rest is greenish ; the small 

 irregular area with an extinction of 18 is yellowish green and distinctly pleochroic. 



"A typical fresh specimen of the porphyritic granite (No. 86G) was powdered 

 and the augite separated and analyzed. This augite is fresh and unaltered and the 

 powder used (which has a higher specific gravity than 3) is quite pure, as in this 

 specimen of the granite the only other minerals present were feldspar and quartz 

 with a few minute fibres of secondary hornblende. The analysis is here given: 



A in i lysis of uni/il<'. 



SiOj A1 2 O 3 Fe 2 O s FeO CaO MgO K 2 O Na 2 O H 2 O Total 



53.19 2.38 9.25 5.15 17.81 9.43 0.38 2.03 0.01 100.23 



" Assuming that this represents an isomorphous mixture of the diopside, hedden- 

 bergite, acmite and fassaite molecules and calculating their relative proportions, we 

 get approximately the following result: 



Diopside, Mg Ca Si 2 O,, 47 percent, 



Heddenbergite, Ca Fe Si a O fi 27 " 



Acmite, Na Fe Si 2 O, ; 21 " " 



Fassaite, Mg A1 2 SiO 6 5 " 



" Iii the considerable percentage of the acmite molecule this augite approaches in 

 composition the pyroxene of the more alkaline rocks, the eleolite syeuytes.* This 

 analysis very probably represents quite well the usual composition of the green 

 augite, as the proportion of zonal crystals, with colorless centres and entire colorless 

 crystals is small. The colorless augite is very similar to that of the well known 

 augite granite from Laveline in the Voges." 



An analysis of the whole rock gave the following results: 



SiO, 07.42 



A 1.0, 15.88 



!'< O. - 1.157 



FeO 1.14 



MnO trace 



CaO :;.!!) 



MgO 1.43 



K 2 o.,;,-, 



Na 2 O (i.|-j 



P 2 6 ('.07 



H 2 0.05 



Total, 



*Cf. A. MKHIAN. Sludicnan aettcinsbildcHdenPyrojceiuii; ffeuei .In I, i-lnu-li /. Mix., Pet, u. Pal., B.B. Ill, pp. 252-315, 1885. 



