ORIGIN OF THE NODULAR STRUCTURE. alefale 
drop-like forms which the isolated portions of such a liquid would take, 
but which could not be developed ina magma when crystallization was 
far advanced. ‘The constituent minerals of the nodules furthermore are 
not identical with the last formed constituents of the granite, as they 
should be if the nodules represented the last products of the crystalliza- 
tion of the granite magma. Microcline, which is abundant and one of 
the last minerals to separate out of the magma in the case of the granite, 
is entirely absent from the nodules, while sillimanite, which is never 
found in the granite, is one of the most abundant constituents in the 
nodules as well as one of the first of the constituents to crystallize. ° 
The peculiarity of the present occurrence lies chiefly in the fact that 
the portion of the magma which thus separated out was more acid than 
the magma as a whole and richer in alumina, which is very unusual. 
It must be remembered, however, in this connection that the granite 
itself is more acid than usual. 
Maegmatic differentiation has been put forward by Backstrom to account 
for the origin of the nodular granite found at Kortfors, in Sweden,* in 
which, however, the nodules are more basic than the granite itself, as 
well as to account for other allied occurrences. He thinks that ‘‘it is in 
many cases evident that the inclusions were soft, and then the simplest 
view is that they were drops or portions of a partial magma, which, at 
the temperature existing immediately before crystallization, could no 
longer be held in solution by the principal magma but separated out.” + 
This seems to be the only satisfactory explanation of the Pine Lake 
occurrence, the history of whose development seems to have been as fol- 
lows: In the original magma there were certain “schlieren” richer in 
silica and alumina than the rest of the magma, and containing also a 
certain amount of boracic acid. How these came into existence, whether 
by segregation or separation from the immediately surrounding magma, 
or whether brought into their present position from another part of the 
mass by movements in the molten magma, is uncertain. We have, how- 
ever, examples of such differentiation in granite magmas in the case of 
pegmatite veins, which at their extremities frequently run out into veins 
of quartz associated with a little tourmaline. These schlieren, being 
evidently immiscible with the main mass of the magma, were analogous 
to globules, streaks, or sheets of oil in water, except that the magmas, 
being much less mobile than these fluids, the schlieren could not so readily 
run together into globules or rounded masses; or they might be com- 
pared with the globulites which separate from a solution which is about 
to crystallize and which, after appearing separately, aggregate themselves 
together into rows like strings of beads and eventually develop into va- 
* Tvyenne Nyupptackta Svenska Klotgraniter. Geol. Foren, i. Stockholm Foérh. 1894, p. 128. 
+ Helge Bickstrém : Causes of magmatic differentiation. Journal of Geology, vol, i, 1893, p, 778, 
XXYV—Bur1, Gor, Soc, Am., Vou, 9, 1897 
