214 ALEXANDER N. WINCHELL 
necting links between alkalcic and alkaline rocks. It is especially 
important because it constitutes large intrusions in California and 
elsewhere. 
The volcanic equivalent of granodiorite, which may be appro- 
priately called rhyodacite, forms another intermediate group 
between the alkalcic and the alkaline rocks. Having no distinctive 
name available, rhyodacites have been classed with dacites in the 
past; the two groups differ as distinctly as tonalite and grano- 
diorite. It is because the rhyodacites and even some quartz latites 
have been described as dacites in the past that Daly" finds the 
average composition of dacite corresponding to that of grano- 
diorite rather than tonalite. As may be seen from the tables of 
average composition of igneous rocks on pp. 217-221, dacite, like all 
the volcanic rocks, is more siliceous than the corresponding plutonic 
rock, tonalite, but is nevertheless its equivalent, as indicated 
especially by the tenor of alkalies. Similarly rhyodacite and 
granodiorite are related. 
A rock which contains more ferromagnesian minerals (and less 
silica) than granite, but is nevertheless characterized by dominant 
alkali feldspar and quartz with subordinate plagioclase, has been 
called quartz syenite by Brogger.? It is sometimes regarded as 
intermediate between syenite and granite. But it is an alkaline 
and not an alkalcic rock type, and is very closely associated with 
quartz monzonite in the field. It should be regarded as a potassic 
variation from quartz monzonite, and not as a type closely related 
to granite. It is surprising to find that the average quartz syenite 
is less siliceous than the average quartz monzonite. This may be 
due to a tendency to classify the more siliceous quartz syenites as 
granites. 
The relative depth at which rocks crystallized is estimated 
commonly by a study of their texture. Thus, plutonic rocks have 
granitic texture, volcanic rocks usually have felsitic or porphyritic 
texture; and textures are independent of mineral composition. 
But the ophitic texture, characterized’ by the crystallization of 
R.A. Daly, Proc."Amer. Acad., XLV (1910), 239. 
2W. C. Brogger, Zt. Kryst., XVI (1890), 81. 
3A. N. Winchell, Bull. Geol. Soc. Amer,, XX (1908), 661. 
