SYMBOLS IN CLASSIFICATION OF IGNEOUS ROCKS 761 
a given rock falls, also the number of the division toward which it is 
transitional. 
The intermediate position of a rock is expressed by prime 
marks placed either before or after the number of a division accord- 
ing to the direction in which the rock is intermediate. 
The result of applying these devices to the quantitative classi- 
fication of Clarke’s average rocks is shown in the accompanying 
table. It appears at once from the symbols of this table that all 
but three of the average rocks fall in the central half of Class IT; 
that the three exceptions are intermediate to Class I, and that one 
is within the transitional bounds. The ratios show that not one 
of these rocks is half-way from the center point of Class II toward 
Class II. , 
QUANTITATIVE CLASSIFICATION OF AVERAGE ROCKS 
Class | Order| Rang Subs 
Average of Analyses Name Sal Q Alkalies Ko" 
Fem F CaO’ Na.0’ 
H._ 137 rocks of Colorado....| Dacose-adamellose GDIOL |) AAC 2 oy 3(4) 
A 248 analyses, Roth’s tables} Adamellose-dacose MOE pay 2(3) | ()4 
G 113 rocks of Yellowstone . 
TREY ke aaa ede Se Ue Harzose-tonalose II | 4(5) | (2)3 (3)4 
oe North America 455.55: Harzose-tonalose II | 4’ (2)3 (3)4 
E U.S.G.S. analyses....... Harzose-tonalose IES a '3 (3)4 
B_ 536 British rocks, Harker | Harzose-tonalose II | 4 3 (3)4 
O 1659 rocks, Europe...... Harzose-tonalose IT | 4(5) | (2)3 (3)4 
Q World’s average, Clarke. -| Harzose-tonalose TI | 4(s) ie (3)4 
I 1095 rocks of California. ..| Tonalose II | 4 3 4 
P 82 rocks of South America| Tonalose Te eae 3 4 
N_ 250 rocks of Italy....... Shoshonose-monzonose IT | (4)5 | 2(3) 8 
M 420 rocks of Germany and 
PAIS ERA sina sehr aor eee Shoshonose-andose TA (Aisalna(@\2 (3)4 
F 250 rocks of Atlantic 
Coast Wis eee Shoshonose-andose IT | (4)5 43 (3)4 
C World’s average, Wash- 
LEON esi lae aia ine aan Monzonose-akerose Mays | 23) Ga 
L231 rocks of northern 
ISU) NSa wig cok eu dog me Akerose Au Sulu ‘4 
With regard to the relation of normative quartz and feldspar, 
determining the order, it is seen that eight rocks fall within the 
narrow transitional limits between Orders 4 and 5, half of them on 
one side and half on the other. Of the remainder all but two are 
intermediate toward the same boundary line. Of the two rocks 
coming within the central half of Order 4, B and I, the former is 
