658 The American Naturalist. [July, 
The Petrography of Aegina and Methana.——The lavas of 
the island of Aegina and the peninsula Methana in Greece are ande- 
sites and dacites that have broken through cretaceous and tertiary 
limestones. Washington’ separates the rocks into the two groups 
above-mentioned on the basis of the SiO, contents. Rocks containing 
above 62% of SiO, he classes as dacites, those containing less than this 
amount as andesites. The dacites are divided into hornblende, horn- 
blende-hypersthene and biotite varieties, and the andesites into horn- 
blende, biotite-hornblende, hornblende-augite, hypersthene and horn- 
blende-hypersthene varieties. All the rocks are more or less porphyritic, 
and all contain more or less glass. Tridymite is present in the horn- 
blende andesites from the Stavro district. The trachyte described by 
Lepsius from near Poros is a biotite-hornblende-andesite. Brown and 
green hornblendes are both present in the Grecian rocks, but not in the 
same specimens. The green variety is characteristic of the pyroxene 
free andesites, and the brown variety of those rocks containing an almost 
colorless pyroxene as one of its essential components. This association 
of the two hornblendes indicates that their formation is dependent upon 
differences*in chemical composition of the magmas from which they 
separated, as well as upon the conditions under which their separation 
took place. 
In almost all of these rocks there are segregations of the same com- 
position as that of the enclosing rocks, except that they are more basic. 
Two classes of segregations are observed. The first are hornblende- 
augite-andesites, containing brown hornblende and no glass; the second 
class is composed of green hornblende in a glassy base with plagioclase 
laths. The brown hornblendes are often changed to opacite, surrounded 
by a zone of colorless crystals of augite. In those segregations in which 
the hornblende is of the green variety, nosuch alteration is observable. 
The glass in these segregations is so different from that of the rock in 
which they occur, that it cannot be regarded as portions of the latter. 
The author is inclined to regard these bodies as fragments of earlier 
lava flows buried deeply beneath the latter ones. 
In his discussion on the general relations of the different rocks of the 
region, the author states that “in general * * * the more acid the 
rock the more vitreous the groundmass, the smaller and more micro- 
litic the crystals in it, and the larger and more abundant the pheno- 
cysts.” 
After remarks on the chemical relations of the different rock types 
to each other, and a discussion of the Aegina-Nisyros region as a 
5 Jour. of Geology, Vol. II, p. 789, and Vol. III, p. 21. 
