302 Classification of Massive Rocks. 
Their structure varies widely in consequence of the fact that they © 
occur at considerable depths within the earth, and are moreover 
very sensible to chemical alteration. They are separated, according 
to their structure, into :— 
(A) holocrystalline dacites, with many or few porphyritic 
constituents. 
(B) felsodacites, with a microfelsitic ground mass. 
(c) andesitie dacites, with a hyalopilitic ground mass. 
(D) vitrophyric dacites, or dacite glasses, with the compo- 
sition of dacite holding porphyritic crystals of pla- 
gioclase, quartz and the iron-bearing silicates. 
E. THE ANDESITES. 
The andesites are neovolcanic rocks, composed principally of pla- 
gioclase and the iron-bearing silicates of the biotite, amphibole and 
pyroxene groups, thus corresponding to thé porphyrites of the palæo- 
voleanic group, and the diorites, and some of the gabbros of the 
intrusive class. 
They are divided, according to: the prevalence of one or the other 
of the iron-bearing silicates among the porphyritic constituents, 
into :— 
1, Mica—ANDESITES, in which biotite predominates over the other 
iron-bearing minerals. They are subdivided, 
according to the structure of their ground mass, 
into :— 
(A) holoerystalline mica-andesites. 
(B) felsodacitic mica-andesites. 
(c) mica-andesites proper. 
(D) trachytic mica-andesites. - 
(£) vitrophyrie mica-andesites. 
2. HoRNBLENDE-ANDESITES, in which hornblende predominates. 
These are subdivided as are the mica andesites. 
3. AUGITE-ANDESITES, in which a monocline augite is the prevail- 
ing iron-bearing porphyritic constituent. Subdi- 
vided like the mica-andesites. 
4. HyPERSTHENE-ANDESITES, in which a rhombic instead of a 
monoclinic augite is the prevailing porphyritic con- 
stituent. Subdivided like the mica-andesites. 
5. HyALOANDESITES, or ANDESITE GLASSES, glasses with the com- 
