Sortas—On Pitchstone and Andesite. 93 
Numerous more or less spherical inclusions occur, which are 
evidently infilled vesicles. In some cases these are bordered ex- 
ternally by felspar laths, lying tangentially, an arrangement 
probably resulting from the expansion of a bubble of steam, 
driving the glass and included felspar laths before its advancing 
walls (fig. 4). The infilling material differs in different cases; 
sometimes it is calcite, sometimes deeply reddish coloured glass, 
occasionally opal. 
Fie. 4. 
The complete absence of even incipient devitrification in these 
glassy rocks forbids their reference to any earlier date of eruption 
than the Tertiary; rocks of remarkably fresh appearance are 
known from much more ancient periods, but these are crystalline 
—never, so far,as I know, truly vitreous. 
