ROCK CLASSIFICATION ON THREE CO-ORDINATES — 213 
dominant over felsic minerals including lenads, and, third, those 
containing no essential feldspar nor feldspathoid. Subdivisions are 
based largely upon quartz and olivine, since these minerals serve 
to measure the relative amount of silica present—the presence of 
quartz or the absence of olivine indicating a higher relative silica- 
tion than the reverse condition. 
It may be desirable to state explicitly that there are igneous 
rocks which will not readily find a place in this classification just 
as there are rocks which do not fit each other classification. Such 
a condition must exist, since rocks exhibit all sorts of gradations 
from one type to another, and classifications, on the other hand, 
establish sharply separated categories. A few illustrations will 
serve to emphasize this fact. 
Rocks are classified as plutonic, hypabyssal, and volcanic, 
depending chiefly upon the depth at which they consolidate. But 
it is clear that there is a complete gradation in depth from the 
surface to the greatest depths open to observation. Similarly 
there is a gradation in the rocks formed at various depths. Hypa- 
byssal rocks are found in dikes (or sills) consolidated at moderate 
depths. The same dikes may contain plutonic rocks at greater 
depths and volcanic rocks near the surface. Only those dike rocks 
which differ in some recognizable way from the plutonic and 
volcanic rocks are included as hypabyssal rocks. 
Again, rocks are classified as alkalcic and alkaline. But there 
are many gradations from one type to the other. Granodiorite is 
an intermediate group of considerable importance. Assuming a 
total feldspar tenor of 60 per cent, Lindgren’ has defined tonalite 
(or quartz diorite), as containing less than 8 per cent of alkali feld- 
spar, granodiorite as containing 8-20 per cent of alkali feldspar, 
quartz monzonite, as containing 20-40 per cent of alkali feldspar, 
and granite as containing more than 4o per cent. The percentage 
of alkali feldspar cannot be accurately determined from an analysis 
of the rock because potassa (K.O) may enter the plagioclase and 
soda may enter the orthoclase; it should be obtained by direct 
microscopic measurements. Granodiorite is intermediate between 
tonalite and quartz monzonite, and is therefore one of many con- 
™W. Lindgren, Amer. Jour. Science, IX (1900), 269. 
