146 LTE OOLTEN ALES OLS GHA OLE OGN 
individuality and not being completely disintegrated during the 
violent movement of translation by the lava stream from a pre- 
sumably great depth to the surface if some solid binding material 
did not exist between the large component crystals, we must 
suppose this interstitial matter to have solidified as glass (or 
trachytic groundmass) at a great depth. This groundmass, 
whether vitreous or holocrystalline, differs radically from that 
of the surrounding rock and so cannot be supposed to be due to 
penetration of the enclosure by the still liquid magma in which 
it lay. While such enclosed masses have been classed here and 
by others as endogenous enclosures in the sense already defined 
(p. 138), yet the presence of such a glass base tends to throw 
doubt on this view and makes it seem possible that we have here 
to do with fragments of an earlier lava flow, at present buried 
deep beneath the later superincumbent lava streams. This 
question will, however, be discussed at greater length later. 
STRUCTURE AND MINERALOGICAL COMPOSITION. 
Before we begin the chemical discussion of these rocks it 
will be as well to speak of one or two points in regard to the 
general structural and mineralogical characteristics of all the 
rocks described from this region. As regards structure, it may 
be said that megascopically the most basic rocks—such as 
the hypersthene-andesites and from a certain standpoint the 
segregations——are the most even-grained and show fewest pheno- 
crysts, a porphyritic structure being more and more highly 
developed, and the size and abundance of the phenocrysts 
increasing, 
the whole standing in this respect (as well as chemically) at 
as a rule, with increasing acidity; the dacites on 
the opposite end from the Oros hypersthene-andesites. The 
chief exceptions to this general rule are most of the horn- 
blende-augite-andesites of the Monastery District. To put it 
broadly, the difference may be expressed by saying that basic 
rocks are largely groundmass, while the acid rocks are to a much 
larger extent phenocrystic. Microscopically it is seen also that 
the groundmass of the dacites, while less abundant, is much 
