Gabbroid Rocks of Minnesota. — WincJiell. 353 
The ORIGIN of this rock is not as clear as that of the preced- 
ing, but the presence of a large amount of quartz indicates that 
it is the result of the absorption by the normal gabbro of 
clastic siliceous rocks. It is derived, indeed, from the north- 
ern extremity of the area of gabbros where the latter were 
in contact with various older rocks; the quartz gabbro is as- 
sociated with the noryte. 
Chapter IX. Silicoferrolyte. 
The name ferrolyte was first proposed h\ \\'adsworth* 
in 1892 to designate a rock essentially composed of iron ores. 
In this chapter will be studied a rock extremely rich in 
magnetite, and it is proposed to employ the term silicofer- 
rolyte to designate it. 
It contains, in fact, a remarkable quantity of quartz, and 
of a ferruginoussilicate, and it seems preferable to leave to 
the term proposed by Wadsw-orth its more general significa- 
tion. Silicoferrolyte is the rock designated "olivine iron ore'" 
by the Minnesota Geological Survey. It is. indeed, composed 
essentially of magnetite, fayalite and a small quantity of 
pyroxene; these three minerals enclose numerous small grains 
of quartz. The magnetite is in such quantity as nearly to con- 
stitute a paying iron ore. But in the state which leads the 
world in the production of iron of very high grade, low grade 
ores naturally pass unnoticed. 
The sample to be studied came from the north side of 
Birch lake, near the northern edge of the gabbro area. Ac- 
cording to N. H. Winchell silicoferrolytes pass into gabbros 
l)y insensible gradations. Occurrences of a similar rock are 
known farther east along the northern border of the gabbro, 
and others occur near Duluth. and at points well within the 
gabbro area. 
Silicoferrolyte often presents a rough schistosity which, 
may sometimes be seen even in hand specimens. The color 
of the rock is that of the dominant mineral, namely, black, 
with bright metallic lustre on fresh surfaces. Occasionally 
the rock is greenish black, when the fayalite is abundant. 
Magnetite and fayalite are the only minerals distinguishable 
*M. E. Wadsworth: Rept. State Gcol. Mich. 1891-92. p. 92. Ret. 
Kemp: Handbook of Rocks. i(S96. p. 1,37. 
