MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 251 



had such considerable size as to impede the equalization of Ab : An 

 in the solid and the liquid phase. 



As another example we refer to the orbicular quartz-norite 

 from Romsaas (described in this Journal for 1921, pages 430-35), 

 the plagioclase, here also disregarding the small quantity of Or, of 

 an average composition of Auj Abi (or Auga Ab48), with the kernel 

 of Augg Ab42 and the exterior of An45 Abjs, locally even up to Anjg 

 Ab62. This orbicular norite occurs in a quite small eruptive field 



^Ab. 



lOOArv 

 OAb 



&oAn 

 20Ab 



eoAn 

 4(iAb 



Fig. 56 



MAn 

 60Ab 



20An oAn 

 QOAb lOOAb 



close to the surrounding gneiss, which caused the time of cooling 

 to be somewhat quicker than is common to deep-seated rocks. 



Further I refer to the description given by V. M. Goldschmidt^ 

 of the " Trondhjemite " generally containing 69-76 per cent SiOz, 

 16-13 per cent AIO3, 3-1 per cent FeaOj+FeO, 1-0.2 per cent 

 MgO, 3.5-1.7 per cent CaO, 6.5-4.5 per cent NaaO, 2.5-0.5 per cent 

 K2O, etc. The kernel of the plagioclase shows 20-32.5, most 

 commonly 24-29 per cent An, and the outer zone about 16 per 

 cent An, often with recurrence of basic and acid zones. That the 

 zonal structure of the plagioclase of this rock is more pronounced 



^ Geologisch-petrographische Studien im Hochgebirge des siidlichen Norwegens 

 (Kristiania: Vidensk-Selskob, 1916). 



