666 /. H. L. VOGT 



cent quartz, and a little rutile and apatite, the individualization 

 began with the crystallization of hypersthene in large quantities. 

 Then followed, during a short stage, a simultaneous crystallization 

 of hypersthene and biotite, while the Mg, Fe-silicate present in the 

 magma during the last stage entered totally into the biotite. 



The anhydrous meta-silicate hypersthene thus during the later 

 stage of the crystallization — even after the percentage of SiO^ in 

 the residual magma was quite considerably increased, viz., to about 

 6 1 per cent — was replaced by the hydrous ortho-sWicaXQ biotite. This 

 must be due to the fact that the original percentage of H2O of the 

 magma was so small, that at first hypersthene could be formed. 

 But when a considerable quantity of this mineral was segregated 

 H2O became so strongly concentrated that biotite also could be 

 formed. And during the last stage, when the remaining mass was 

 reduced to ca. -^^ of the entire rock, the quantity of H2O was thus 

 very considerably increased, so that the formation of hypersthene 

 ceased, and biotite was formed instead. 



As we shall treat more particularly in a later paper, there occur, 

 in the quartz-norite massive at Romsaas, a number of pegmatitic 

 " Schlieren" and dikes of nearly the same mineral composition as the 

 intervening mass between the orbs of the orbicular norites, but 

 with the plagioclase (AbegAuja) somewhat richer in Ab and with 

 somewhat more quartz. These pegmatitic "Schlieren," etc., 

 must represent the end-magma, resulting from a very late stage 

 where the quantity of H2O was still more concentrated. This is in 

 accordance with the pegmatitic structure and moreover with the 

 fact that the Mg, Fe-silicate here only enters into biotite, while 

 hypersthene is entirely lacking. 



In several norites from Norwegian localities examined by me, 

 biotite is entirely lacking in almost all thin sections from one single 

 field (Ertelien on Ringerike) while a little biotite occurs in most 

 fields, mostly 2, 3, 4, or 5 per cent, and only as a rare exception, as in 

 the quartz-norite from Romsaas, as much as 8 per cent. Where 

 both hypersthene and biotite occur, the last one always, as just 

 described from Romsaas, belongs to a somewhat later stage than 

 the hypersthene. It is especially to be emphasized that neither 

 the absolute quantity of biotite nor the quantitative proportion 



