280 HENRY S. WASHINGTON 
magma solidifies under sufficient pressure, as in the Lausitz 
stock; or where it cools so quickly that time is not given for 
alteration, as in the Auvergne dikes or the borders of the Mount 
Cimino streams. They undergo more or less alteration (and are 
hence replaced by augite) when such time is given them while 
they are simmering under slight pressure in the throat of the 
volcano, as seen in the lava flows of Auvergne and the Lausitz, 
In the case of the Mount Cimino flows we may suppose that the 
slow cooling of the interior under slight pressure gave oppor- 
tunity for the replacement of the biotite by augite. 
There are certain other cases where it seems possible that for 
some reason the magma was brought near the surface before the 
formation of hornblende or biotite had taken place to any extent, 
so that the crystallization of augite out of the magma took place, 
instead of either of the two other minerals. 
As an illustration of this may be mentioned the volcanoes of 
Santorini. Here the highly acid pyroxene-andesites ejected by 
the later eruptions which formed the small island cones in the 
center of the bay show in the abundant glassy groundmass a large 
preponderance among the augites of well-formed prismatic micro- 
lites, as well as abundant hypersthene prisms, neither of which 
could be due to the alteration of hornblende, while augite grains 
are quite rare. The lava flows which go to make up the external 
crater ring of Thera and Therasia are in general much more 
crystalline and at the same time contain many more imperfect 
grains of augite, as I gather from examination of specimens col- 
lected by myself, and from some of Fouqué’s remarks.’ In this 
connection it is of interest to note that the hornblende crystals in 
the hornblende-andesites of Acrotiri very seldom show alteration 
phenomena.’ 
These explanations may seem forced or unnecessary to many, 
but they are only offered as suggestions of the applications of 
the idea of the instability of hornblende and biotite under erup- 
tive conditions to certain problems of petrology. The forces 
tFOUQUE, Santorin, Paris, 1879, 291, e¢ ai. 
2 FOUQUE, Santorin, 357. 
