24 IeI3 J OXUMAUIL (QU CIB OVLI OG. 
made up, to a large extent, of colorless or very pale brown glass 
base, this being much more abundant in the Spasmeno rock than 
in that from Stavro. In this base are strewn quite abundant 
plagioclase lathes" and small colorless microlites, some of which 
seem to be augite, many small magnetite grains, and (especially 
in the Stavro rocks) much brown ‘‘dust”’ in streaks and patches. 
A flow structure is developed among these smaller constituents, 
which is especially well marked in the Spasmeno rocks, where 
also the ‘“‘dust”’ is almost entirely absent. There are also to be 
reckoned among the groundmass constituents a number of small 
plagioclase crystals and green hornblende prisms. 
The discrimination between the phenocrystic and the ground- 
mass plagioclase is quite easy, as the latter is almost always in the 
shape of small clear lathes, while the former is in much stouter 
crystals and generally with inclusions. The plagioclase pheno- 
crysts are of fair size, and, under the microscope, seem quite 
fresh and unaltered. The stout crystals show the usual planes 
and very few fracture surfaces are to be seen. Twinning lamel- 
lee, in nearly every case according to the albite law, are common 
and the extinction angles of 29°-30° 30’ on ¢ (oor) indicate 
that we have here a bytownite of about the composition Ab, JEN 
though in all probability other members of the series are present. 
Such a basic plagioclase is unusual for hornblende-andesite 
whose feldspar is generally labradorite or andesine, and I very 
much regret that the lack of proper facilities in Venice, where 
this paper was written, prevented the examination of the feldspar 
by other methods. The small lathes also belong to the bytow- 
nites, but at the end of the series richer in albite. Zonal struc- 
ture is quite frequent. 
Inclusions are very common in the larger crystals and in the 
great majority of cases are of generally clear brownish glass, 
occasionally dusty, and frequently holding a bubble. In many 
cases these glass inclusions are so numerous that they form a 
«The term “lathes” is used for the larger, but still small, long groundmass crystals 
{especially of feldspar), while microlites are the smallest in size and in many cases of 
an indeterminable nature. 
