696 HENRY S. WASHINGTON 
Megascopic—The rocks are compact and generally a very 
light gray, almost white, though a flow at Cuddia Nera is light 
pinkish through weathering; small phenocrysts of feldspar are 
numerous, with fewer of black hornblende and augite. The 
groundmass is very fine grained, somewhat dull, and phanero- 
crystalline. 
Microscopic.—The largest phenocrysts are of soda-orthoclase, 
which are tabular parallel to 6 (o10), and are like those already 
described. More interesting are those of hornblende which form 
not more than about 2 per cent of the rock. These are euhedral 
to subhedral, slender prisms, about 0.5 mm. long by o.2 mm. 
thick. They have the faces m (110) and 6b (orto) well developed, 
but no terminal planes. The prismatic cleavage is good. When 
these phenocrysts occur in the groundmass they are almost 
invariably surrounded by a narrow border of finely granular 
hornblende of somewhat lighter color, which is absent from the 
few included in feldspar phenocrysts. 
The color of the hornblende is a rather dark chestnut an, 
without the tinge of red which is so characteristic of the cossyrite 
of the various types of pantellerites. The pleochroism is very 
strong; ¢ and & dark brown, or almost black, a light yellow 
brown. It was difficult to determine the extinction angle accu- 
rately owing to the intense absorption, but values of r/c up to 
20. were observed. Though the pleochroism differs from the 
normal cossyrite, the other characters, as well as the chemical 
composition deduced from the norm, show that this hornblende 
is a cossyrite and not a kaersutite which it somewhat resembles. 
A few subhedral phenocrysts of a bright-green, slightly pleo- 
chroic aegirite-augite are present, and in a few sections rare anhedral 
olivine phenocrysts. 
The groundmass is holocrystalline and is mostly composed of 
alkali-feldspar and quartz. The former is usually in the form of 
minute tables, giving rise to lath-shaped sections, though some 
anhedral grains occur. The quartz appears as small patches, 
about 0.1 to o.2 mm. in diameter, usually irregular in outline but 
sometimes roughly rounded, which include feldspar grains micro- 
koikilitically. Scattered abundantly through this quartz-feldspar 
