40 THE JOURNAL OF GEOLOGY. 
composed largely of small angular fragments of the hornblende- 
augite-andesite of the Monastery district, with pieces of the 
brown hornblende, colorless augite, and plagioclase that occur in 
these rocks, this fragmentary material being cemented by a clear, 
colorless isotropic substance. Since chemical analysis (cf No. 
16, Part III.) shows that this tuff is very acid, containing 70.81 
per cent. of silica, and since the rock of which the breccia frag- 
ments are composed contains only about 55 per cent. of silica, it 
seems certain that this glassy cementing substance is a form of 
-amorphous silica, such a silicification of eruptive rocks being a 
not at all unusual phenomenon. This tuff is also, like the preced- 
ing, of subaérial origin. Through a misunderstanding no slide 
of the gray enclosures was made, but, as they present the mega- 
scopical characteristics of the Kakoperato dacite, they may be 
safely classed as analogous to these, though an analysis giving 
71.49 per cent. of silica shows that they also have been subjected 
to the silicifying process. 
The only other eruptive rocks found on A‘gina were a block 
of a hard, greenish, aphanitic hornfels-looking rock which on 
examination proved to be a porphyry; a small rounded loose 
block of a coarse-grained biotite-granitite, with rather gabbro- 
like structure, and which probably came from one of the Cyclades 
as ballast; and many rounded fragments of white pumice found 
on the beaches of A‘gina and Methana which undoubtedly have 
floated from Santorini, as they present the characters of this 
pumice, which I found on my visit there floating in large quan- 
tities in the inner bay of the island group. 
HENRY S. WASHINGTON. 
(To be continued.) 
