160 LTE, JOOTINAL OP GH OL OG Vs 
dark lava, and similar objects made of the same rock were dis- 
covered during the excavations of the American School at the 
Heraion near Argos. The rock is dark gray in color, fine-grained 
and finely vesicular and shows a few small phenocrysts of horn- 
blende and plagioclase, with one or two larger quartz grains. 
Under the microscope it is seen to be a hornblende-andesite with 
a hyalopilitic groundmass of colorless glass base and numerous 
plagioclase microlites showing flow structure, the rather abun-_ 
dant phenocrysts being chiefly yellowish brown hornblendes, 
altered opacitically on the borders, with some clear colorless 
augites and large plagioclase crystals, many showing a “net- 
work”’ core. 
Microscopically it has a certain resemblance to some of the 
A‘ gina rocks, though neither megascopically nor microscopically 
is it identical with any of the rocks seen or collected by me at 
Afgina, Methana or Santorini, nor does it correspond to any of the 
descriptions of Doelter or Ehrenburg of the rocks of Kos or 
Milos. It seems then almost certain that it does not come from 
any of these islands. Now, Strabo’ informs us that the island of 
Nisyros supplied the neighboring region with excellent mill- 
stones, so that it seems reasonable to suppose that the corn- 
grinders found at Plateeaa and Argos (which were Strabo’s mill- 
stones) came from Nisyros. If this be so it would go to show 
that Nisyros has been ejecting much the same sort of rock as 
the volcanoes further west on the same line, which is @ priori to be 
expected. It is needless to remark that the above argument is 
of very little weight, and it is chiefly inserted to emphasize the 
slightness of our knowledge of the Nisyros rocks. 
However, leaving aside for the present the other volcanic 
centers of the fracture line, let us examine more closely the rocks 
of our present region. For this purpose I have constructed the 
diagram of the molecular variation of their constituents as shown 
in Diagram 1. The molecular amounts of silica are given by the 
abscissas, the limits of the diagram extending from .850 to 1.150. 
The iron oxides are given as FeO. In the upper part the six 
*STRABO: Geogr., X. 5-16. 
