Gabbroid Rocks of Minnesota. — Winchell. 3 8 1 
(Ca + Fe + Mg), and Mg < Ca + Na + K. They belong to 
the fourth class of Lang, having a predominance of CaO over 
Na^O + K2O ; and to the type having Na^O > K:=0 < CaO 
< 3Na-0 + 3K2O. They possess neither the characteristics 
of his Diorite-Diabase tNpe nor of his Gabbro-Dolerite type. 
These three types present the characters of the group called 
'"melanokrate" by Brogger * ; that is, they have a predomin- 
ance (by weight) of the ferromagnesian silicates, and belong 
to the calcic sub-group ("calcicoplete"). 
The plagioclasyte (Fig. 7) shows evidence of its purity as 
simple labradorite feldspar by the absence of the ferromagne- 
sian triangle ; indeed, the calcium excess no longer exists ; the 
rock is saturated with alumina, but the excess of alumina 
(rt'-= .09) is too slight to permit representation. The triangle 
of basic elements is thus reduced to the very short ferro-mag- 
nesian line. The ratio K^^O : Na-O has noticeably changed, 
and is nearly i : 10; this is due to an increase in the sodium 
without a corresponding increase in the potassium. It is in- 
teresting to note that this plagioclasyte belongs to the granitic 
magma of Michael Levy, being rich in calcium( and alumina ; 
a — o, c ' = o), and poor in magnesia and potassium. On the 
contrary it is derived from the gabbro magma of Rosenbusch 
since Ca > Na + K, while Mg + Fe <Ca + Na + K and Si 
< Ca + Fe + Mg. According to Lang's classification this 
rock belongs with the three preceding types. On the other 
hand it represents Brogger's second grand division, namely, 
the "leukokrate," since the alkaline-earthy aluminous silicate 
labradorite greatly predominates. 
The troctolyte on the contrary (Fig. 2) shows a notable de- 
velopment of the ferromagnesian triangle, and since pyroxene 
occurs only in very small amount the ratio /;/ -./very approxi- 
mately indicates the composition of the olivine. The ratio 
f : n shows that the feldspar is again labradorite ; the ratio 
k : n still remains nearly i : 5. This rock also belongs with 
the preceding types according to Lang. It represents the 
peridotyte magma (n-) of Rosenbusch, showing none of the 
characters of his gabbro magma ('/'), though it is certainly a 
gabbro in mineral composition. It seems to represent the 
*W. C. Brogger: Dps Ganggefolge des Laurdalits, 1898. p. 258 el 
seq. 
