282 
The Greenlandic liebenerite-porphyry contains numerous 
phenocrysts of felspar and liebenerite lying in a very fine- 
grained or dense ground mass of a brick-red or brownish-red 
colour. The felspar phenocrysts are light red and occur in 
well defined crystals which are tabular on (010). The thickness 
of the crystals is 1—3 millimeters, the length 1—2 centimeters. 
They are often Carlsbad twins. In the more weathered varieties 
of the rock they may sometimes be picked out of the ground 
mass without difficulty’. Under the microscope they are found to 
be considerably altered, filled with muscovite aggregates and dust. 
In most cases they are too turbid for optical determinations ; 
occasionally, however, they are tolerably transparent in thin 
sections, and the structure then proves ralher inconstant: some- 
times they are soda-orthoclase, sometimes microperthite, and at 
other times microcline with a marginal zone of plagioclase. 
The liebenerite is in well developed and tolerably equidi- 
mensional crystals of the ordinary form characteristic of nephe- 
line. The crystals are seldom larger than 0:5 centimeter. The 
colour is greenish-gray. They consist entirely of a muscovite 
aggregate and no remnants of the original nepheline have been 
found. As shown by the microscopic examination dark minerals 
were originally represented among the phenocrysts of this rock, 
but they have been converted into aggregates of chlorite, cal- 
cite, and iron oxides. 
The ground mass is always very decomposed. Its main 
constituent is an alkali-felspar in small laths. Flow structure 
is not observed. 
' The felspar crystals from the above mentioned 1°5 meter wide dyke have 
been described by 0. В. BOccitp (Meddelelser om Grønland XXXII, 1905, 
р. 440). They are twins, and the twin plane is parallel to (201). The 
forms observed are (010) (001) °(110)* (201). In sections parallel to (010) 
| found the extinction angle to be + 10°; these crystals accordingly are 
soda-orthoclase. 
