690 HENRY S. WASHINGTON 
Mode.—Owing to the fine grain and complex texture of the 
groundmass, an exact estimate of the mode by microscopic measure- 
ment is impossible. When the norm is compared with the thin 
sections the amount of mafic minerals—pyroxene and hornblende— 
present is seen to be much larger than that of the normative 
pyroxene, while but a very small proportion of the normative 
ores is present in the mode. The mode is, therefore, somewhat 
abnormative, and it is clear that readjustments of the norm to 
estimate the mode must consist chiefly in combining the normative 
ores and pyroxenes with some silica and albite. These read- 
justments cannot be made with certainty, as two pyroxenes and a . 
hornblende are present and their composition is unknown. But 
the amounts of these are small and, taking the thin sections into 
consideration, the following will represent fairly well the relative 
amounts of minerals present in my analyzed specimens from 
Montagna Grande A and Zichidi B. 
A B 
Quartz RNC ote lap reine 3 13 
Soda-microcline............ 83 70 
Aepirite-au piles serrate 7 9 
Hornblendesi4 = e- 422. 4.- 5 6 
IMMEANEOS o'4 olo cone oouoes 2 2 
100 100 
These rocks are evidently slightly quartzose soda-trachytes, 
that of Zichidi containing so much quartz as to be transitional 
to the rhyolites. As this, however, is confined to the groundmass 
and is not at all prominent even there, it had best be classed with 
the trachytes. Their sodic character is chiefly evident in the 
feldspars, the augite is only slightly aegiritic, and the amount of 
soda-amphibole (cossyrite) very small, in these two latter respects 
differing from the next type. 
Such alkali-trachytes have been called by Rosenbusch augite 
trachytes of the Ponza type. These of Pantelleria differ widely 
from the true Ponza trachytes, which Rosenbusch has described 
and specimens of which I have studied, in the total absence of 
biotite, either as such or as augite-magnetite aggregates represent- 
