1921. No. 8. 



ON TllK IXI.OGITKS OF XORW.W 



69 



The plagioclase in the ground-mass is drciniillinu'tc r-graincd and 

 shows a marked in\erse zonal structure. Vhv more alhitic cores have 

 rather sharp limits towards tiie more anorthitic boundary-zones. The com- 

 position varies from Ab-.> to Ab^^ (cf. p. 25). 



The hornblende occurs as small prisms of green colour similar to 

 those in other labradorite-rc^cks in this tract. The zoisite toi'ms some- 

 what larger prismatic crystals. 



The nodules are either composed merely of plagioclase or of margarite and 

 wollastonite, and often zoisite, besides smaller amounts of light mica and plagioclase. 



The plagioclase, in the nodules, a|)pears as a hne-grained mass 

 with serrated outlines of the indixiduals. In its composition it is similar to 

 the more anorthitic 

 portion of the plagio- 

 clase of the ground- 

 mass. 



The margarite 

 is megascopicallv of a 

 faintly reddish colour. 

 The scaly indixiduals 

 are embedded either 

 in plagioclase, wolla- 

 stonite or a fine mass of 

 colourless mica. They 

 often are twinned on 

 (001), the extinction 



in sections normal to the cleawage being distincth- ol)!i(|ue (in ma.ximum 6"'); 

 [î = 1-645 i 0.003. J 1^^ negative acute axial angle is of the same order 

 of magnitude as that in many \arieties of muscovite, while the bii'etringence 

 is much lower. Sp. g. 3-024. 1 tried to separate the mineral with hea\y 

 solutions, but did not succeed, much adheiing mica being left. A partial 

 analysis of this material showed a composition conlormal)le to that d 

 some mixture of margarite and muscmite. 



I he wollastonite forms a granuiai" mass of somewhat larger grains 

 than all the other minerals. They are speckled with inclusions of margarite, 

 light mica and zoisite. Diagnostic characters are the clea\-age and a negatixe 

 optic character with 2 \' 45 appr. 



The mica shows the chai'acters of ordinar\- muscoxite. 

 The quantitative proportions of the minerals in the nodules are rather 

 variable, but, on the whole, the margarite is the most abundant in most 

 of them and gives the rock its peculiai" chemit'al character. This mineral 

 contains over 50 percent alumina, and thus the bulk composition also is 

 characterized by a high exess of" alumina, besides a low amount of silica, 

 and much limc". 



It may be asked: how ha\e these nodules originated.-' The presence 

 of wollastonite could suggest the hvpothesis of silicified inclusions of lime- 



Fig. 10. Orbicular labradoritc-rock with lenses ot'eclogite (black) and 

 a transitional rock between eclogite and the orbicular labradorite- 

 rock (black with white dots). .Seljenaesholmen, Selje, Nordt'jord. 



