179 
attain a length of only one-fourth or half a millimeter. They 
never show perthitic intergrowths, but the majority of them 
are pure albite while a few are pure microcline. As a rule 
they are heaped together in small clusters or are arranged in 
definite short trails; very often they are enclosed within the 
arfvedsonite-anhedra. 
Nepheline is very abundant in clear crystals or grains of 
the usual form, averaging two millimeters in diameter. It is a 
little less idiomorphic than the felspar. As a rule it contains 
numerous fluid-pores and flakes of arfvedsonite; less frequently 
egirine-needles occur as inclusions. In many cases the nephe- 
line has been partially converted into sodalite. - 
Ægirine and arfvedsonite are usually present in about 
equal amounts and are always very abundant, all varieties of 
the kakortokite showing a melanocratic character. In the black 
sheets of this rock the total amount of ægirine and arfvedso- 
nite is 65 per cent. In the white variety of the kakortokite and 
in the red one both minerals are allotriomorphic and largely 
interstitial between the light-coloured constituents. In the black 
kakortokite, too, they are less idiomorphic than the eudialyte, 
the felspar, and the nepheline; nevertheless both egirine and 
arfvedsonite show a strong tendency towards idiomorphic de- 
velopment in stout prisms, which attain a length of one or two 
centimeters. In several cases the arfvedsonite has been parti- 
ally converted into acmite-aggregates of the usual appearance 
(see p. 137). 
Sodalite rarely occurs in rough dodecahedral crystals. More 
frequently it appears in formless masses filling small interstices 
between the felspar-crystals. As shown by the direction of the 
cleavage-cracks this interstitial sodalite often belongs to the 
same crystallographic individuals as the sodalite occurring as an 
alteration-product of the nepheline. It is probable therefore 
that the conversion of nepheline into sodalite has taken place 
during the final stages of the consolidation-process. 
125 
