Reviews—Kruptive Rocks of the Kristuania Region. 551 
pyroxene, set in aground mass of pyroxene, apatite, biotite, 
magnetite, orthoclase, and a small amount of nephelie 
pseudomorphs. An exceptional type is present as a dyke rock in 
the Precambrian granite, and contains large barkevikitic hornblende 
individuals, and nodules of olivine-enstatite, enstatite-diopside, 
hornblendite, etc., as endogenous inclusions. 
Sannaite, which fills an explosion crater at Ormen, 7 kilometers 
south-west of the Fen region, is a closely related type, forming the 
end member of the damkjernite-sannaite group. It differs only by 
the presence of a higher content of alkali minerals. The phenocrysts 
are of brown-black ‘hornblende, black pyroxene, and sparing biotite 
in a fine-grained ground mass of alkali-felspar, aegirine, and, in 
smaller amount, of nepheline pseudomorphs (muscovite). 
(7) The latest eruptive rocks of the Fen area are the diabase dykes 
of post-Silurian date, and which are so constant a feature in the 
Kristiania region. 
The geological history of the Fen region as pictured by Brdégger 
may now be shortly summarized, the probable genetic relations of 
the Fen sequence being indicated in the following scheme. 
Sovite Ringite 
Rauhaugite 
Dykes 
ESSEXITE 
p} 
Y 
Damkjernite 
Vee Sannaite 
Hollaite Monee Na,O, A1,O, Tveitasite 
Kasenite AN aaa ae pres eR Fenite 
Urtite 
Hollaite Pegmatite 
Juvite 
| { Jace) 
Kamperite } . 
GRANITE 
CARBONATE | 
ROCKS 
The opening phase in the Fen history was the drilling of an 
explosion crater in the Precambrian granite. The initial magma was 
of basic composition, and probably of essexitic type, confirmation 
of which is provided by the great dominance of the melanocratic 
melteigite, and also by the interpretation of the oldest vibetoite 
as an addition product of a calcite magma and a magma of 
yamaskitic composition (hypermelanocratic differentiation-product 
of an essexite magma). 
