348 The American Geologist. December, i9ou 
MINERALOGICAL AND PETROGRAPHIC STUDY 
OF THE GABBROID ROCKS OF MINNESOTA, 
AND MORE PARTICULARLY, OF 
THE PLAGIOCLASYTES. 
[Continued. See plates in the September and October ^'umbe^^i.l 
By Alexander N. Winchell, Hutto. Mont. 
Chapter VIII. Quartz Gabbro. 
The type of this abnormal rock came from the S. E. y^ 
S. W. '/+ Sec. 12, T. 64-6. near Little Saganaga lake, a lo- 
cality situated at the northern edge of the gabbro area. 
The rock is medium to coarse grained, granular to por- 
phvritic in texture, with occasional irregular parallelisms in 
the arrangement of the dark minerals. It is often decom- 
]Dosed, and then can be crumbled rather easily. 
The rock is of a mottled greenish-gray color, usually rather 
light in shade. In the porphyritic varieties the phenocrysts of 
plagioclase with their polysynthetic twinning are usually 
greenish, but occasionally they seem to be reddish, though 
no twinning is then visible, probably on account of decom- 
position. Glassy cjuartz is not very abundant while apatite 
scales are rare. The common dark green mineral resembles 
a pvroxene, but is evidently decomposed. On weathering 
the feldspar first disappears, leaving quartz and biotite con- 
spicuous. The color remains practically unchanged. 
Texture. Under the microscope the decomposition of 
the rock is very noticeable. The feldspar is very often altered 
to a sericitic mass; the pyroxene is entirely transformed to 
penninite with a little biotite. But the texture is still quite 
distinct; it is coarsely granitic with the usual order of crystalli- 
zation, namely, zircon, apatite. pyroxene(?), plagioclase, quartz. 
Magnetite is very rare, or perhaps wanting, as a primary 
mineral. Olivine was also doubtless present, though uncom- 
mon in the fresh rock; it is now represented only by nearly 
amorphous masses of bowlingite. which, however, still retain 
the form of the original, irregularly developed olivine. The 
latter certainly solidified before the plagioclase. and apparently 
before the pyroxene. 
Primary Minerals. Quartz occurs in large irreg- 
