DESCRIPTION OF ROCKS AND ANALYSES 29 
The order of crystallisation is the same as pointed out for the 
phenocrysts, yet the nephelite-forming period closes after the growth 
of the greater part of augite. The last residue is a colourless cement 
with a refractive power much inferior to that of the nephelite and 
of.the balsam; sometimes there are seen tracks of cleavage and 
crystal forms indicating analcite. Belonging to the optical properties 
of the groundmass constituents, they agree well with those of the 
phenocrysts: the olivine always points out a hyalosideritic compo- 
sition, the augite shows hourglass structure, the nephelite is lower 
in refractive power. Among the nephelite sections there are some 
rounded of lower refractive power than the bulk of groundmass 
nephelite and higher than analcite, containing central inclusions of 
magnetite: the dimensions of the sections were too small to test if 
there really is leucite, yet the probability is close. — No traces of 
melilite were tested. 
As accessory mineral great and sharp dodecahedrons of yellowish 
brown perofskite are characteristic; sometimes the crystal forms are 
wanted and it becomes drop formed, with a broad border of black 
magnetite, which is developed on the crystals too. The transition 
between core and border is gradual, by increasing intensity of tint. 
Cleavage cracks [(100) and (111)] are always well developed, the 
mineral is completely isotropic and is wanting inclusions. — No 
signs of weathering or secondary alteration are visible in the rock. 
In the lighter rock, which in mineral composition agrees well 
with the just described one, the perofskite-mineral is more abundant 
and descends in the groundmass too, partly and to great extent 
replacing the magnetite. | 
The slaggy and vesicular scoriaceous rock contains foreign in- 
clusions of pebbles and sedimentary rock fragments. The ground- 
mass in this rock is not differentiated showing dominating brown 
glass. In the dark coloured mass the few square and hexagonal 
sections of nephelite appear as windows. The olivine shows all 
transitions from yellow-brownish pleochroitic substance with the 
