420 WEED AND PIRSSON 
It is an aphanitic, very tough, dense and compact rock of a 
very dark greenish stone color, having a resinous appearance on 
fresh fracture. Numerous white feldsar phenocrysts, which attain 
a maximum size of 10™™, occur scattered through the dark 
groundmass, with rarer small crystals of egirine-augite in stout 
black prisms. Prismatic crystals 3 to 4™™1in length of a light- 
brown translucent mineral occur in the rock, but their nature 
has not been determined for lack of sufficient material. 
Occasional small patches of a brown, metallic-lustered mineral 
are also present, which is shown by its color, by its magnetic 
properties, and a reaction for sulphur, to be pyrrhotite. The 
occurrence of this mineral in an extremely fresh and unaltered 
igneous rock is quite in accord with the rapidly accumulating 
evidence which different observers are furnishing in regard to 
the occurrence of metallic sulphides in basic rocks. 
In thin section the rock shows large phenocrysts of alkali 
feldspar and smaller ones of augite in a fine groundmass, of alkali 
feldspars, egirite,and nephelite. The large phenocrysts of feld- 
‘spar are fresh and of sanidine-like character. They are devel- 
‘oped very thin tabular on (010) and are usually twinned according 
tto the Carlsbad law. An endeavor to obtain definite data 
concerning the optical properties of these feldspars was not suc- 
cessful. The augites are stout columnar crystals of egirine- 
augite with mantles of egirine. They do not show any noticeable 
dispersion of the optic axes. Rarely some rather large crystals 
of zircon occur. 
The groundmass in which these phenocrysts lie is composed 
of irregular, lath-like, unstriated feldspars arranged in a pro- 
nounced trachytoid structure which shows at times a fluidal 
arrangement. Sometimes the feldspars are in short rectangular 
forms. They show the patchy, flamed appearance so character- 
istic of anorthoclase, where the composition varies from place to 
place between the molecules Ab and Or. No albite twinning is, 
however, present, but the majority are singly twinned according 
to the Carlsbad law. Scattered freely through this trachytic 
groundmass are great quantities of rather short, stout microlites 
