SGP INGE! (OUR AE GIINE VAIMID) MIDI IV ECZUINU A. 147 
more vitreous than that of the basic andesites. The largely pre- 
dominating groundmass of the hypersthene-andesites is made up 
chiefly of plagioclase and hypersthene lathes and augite crystals, 
with comparatively little interstitial glass base, while in the 
hornblende-augite-andesites the larger groundmass crystals are 
_ fewer in number (though still numerous), and the rather more 
abundant glass base is very thickly strewn with microlites, mag- 
netite grains and “dust.” 
In the dacites, on the contrary, and in the more acid andes- 
ites, as those of Poros and Spasmeno Vouno, larger crystals are 
comparatively rare in the groundmass, which is highly vitreous 
and often typically, though not densely, hyalopilitic with occa- 
sional perlitic cracks. The tendency in the basic rocks seems 
to be toward forming larger crystals in the groundmass, while in 
the acid rocks the tendency is more toward microlitic develop- 
ment. An excellent illustration of this difference of structure 
as related to chemical composition is seen in the pyroxene-ande- 
sites of Mt. Kouragio and Mt. Chelona, the former containing 
54-53 per cent. of Si@), while the latter contains 59:33 per cent. 
The groundmass of the latter is vitreous containing many micro- 
lites, but few larger lathes, while in the former the glass base is 
relatively much less abundant and the number of larger plagio- 
clase and hypersthene lathes and crystals in the groundmass 
very great. 
So that, in general, it may be said that in these rocks the 
more acid the rock the more vitreous the groundmass, the 
smaller and more microlitic the crystals in it, and the larger and 
more abundant the phenocrysts. To this rule there are, of 
course, exceptions, as seen in the hornblende-augite-andesites, 
since the chemical composition of the magma is only one of the 
many factors involved in the crystallization of rocks.* But inas- 
much as all the rocks of this origin have solidified under quite 
similar conditions as regards pressure and rate of cooling, they 
being all massive (domal) eruptions, and stock, dike or sheet 
*IDDINGS, Crystallization of Igneous Rocks, Bull. Phil. Soc., Washington, 11, 
1889, p. 113. 
