656 FRANK F. GROUT 
COMPARISON WITH OTHER DISTRICTS 
Data from several other districts are given in Table II. The 
method of comparison is modeled after the mathematical statement 
of the quantitative classification.t However, instead of using 
different bases for distinguishing rocks of the different classes, the 
plan in Table II is to use the same criteria for all rocks, making the 
figures strictly comparable through the whole table. The first 
term is the class (I to V) in the quantitative system. The second 
expresses, by groups (1 to g), the ratios of quartz to feldspar, or 
lenads to feldspar. The third and fourth terms similarly express 
the ratio of alkalies to the lime of salic minerals, and the ratio of 
potash to soda, in molecular terms. These are the figures given 
below the norms in Table I of analyses of the Duluth lopolith. 
Scarcity of intermediate rocks——The impression obtained from 
the Duluth series is carried out in the others. There are few inter- 
mediate rocks in any of the districts.. The samples contrast almost 
as Sharply as at Duluth, though none seem to have as complete a 
series of analyses. ‘The main rocks of the associated sequences are 
members of the gabbro family and granites. One rock collected 
by Daly in the Purcell sills is really intermediate, and this is a rock 
which was carefully selected with the idea of showing its transitional 
nature. Probably the Swedish monzonite and some syenites of the 
Adirondacks are also intermediate, but the papers describing them 
are not very definite as to their occurrence and relation. Even if 
the proof of gradational types is clear, as it is at Duluth, the data 
may well be taken as evidence that intermediate rock is less abun- 
dant than the several types on either side of the break. The sharp- 
ness of separation of types in the field has often been noted in 
regions of differentiated rocks.?, To be sure it may be argued that 
a collector would choose clear types rather than a mixed indefinite 
specimen, but the facts at Duluth argue against any such condition. 
Here a good series showing variations of gabbro is available; but 
no series connects the several gabbros with the red rock; and, as 
t Whitman Cross, J. P. Iddings, L. V. Pirsson, and H. S. Washington, Quaziz- 
tative Classification of Igneous Rocks. University of Chicago Press, 1903. 
2L. V. Pirsson and W. N. Rice, ‘‘The Geology of the Tripyramid Mountain,” 
Am. Jour. Sci., XXXI, 2091. 
