233 
and Flink’s Dale) it is the most abundant dark-coloured mi- 
neral. 
Felspar is the main constituent of the rock. It occurs in large 
tolerably idiomorphic plates. The dimensions have been given 
above. Between crossed nicols it is seen to consist of a mi- 
eroperthite with a very fine perthitic structure, often graduating 
into cryptoperthite. In some cases the inner portion of a сту- 
stal is homogeneous or cryptoperthitic while the outer zone is 
perthitic. In sections parallel to (010) the perthitic bands are 
arranged in the ordinary manner at an angle of about 71° to 
the basal cleavage. In sections parallel to (001) the perthitic 
structure is of a reticular type: numerous veinlets of plagio- 
clase follow a direction perpendicular to (010) and are connected 
with one another by ramifications. The structure in some re- 
spects recalls that described from the pegmatites of Narsarsuk!. 
As a rule the potash-felspar of these perthites is orthoclase; in 
some cases, however, an indistinct, gritty or moire twin-struc- 
ture is observed, which indicates the presence of microcline. 
The plagioclase-veins show twin lamellation according to the 
albite law, the lamellæ are fine and the extinction angles 
small. A comparison of the refracting power with that of 
Canada balsam and of nepheline shows that the plagioclase is 
not pure albite, but an acid o/igoclase. Roughly estimated the 
two felspars are present in about equal quantities, but the 
central portion of each crystal is richer in orthoclase while the 
oligoclase predominates in the peripheral portion, and a narrow 
zone of pure oligoclase often surrounds the crystal. More 
rarely crystals of pure oligoclase are found. In a specimen of 
foyaite from the mountain W. of Flink’s Dale the perthitie 
structure differs from the above ‚described type in being coarser 
with relatively large areas of orthoclase and oligoclase which 
are elongated parallel to (010). 
! Meddelelser om Grønland XIV, PI. 4, Fig. 2 (1894). 
