Gabbroid Rocks of Mhmesota. — WinchelL 
365 
A. Replacement of the ferromagnesian minerals one by 
the other. 
1. Pyroxene replaced by olivine. — ^Troctolyte. 
2. Olivine replaced by pyroxene.— Normal gabbro. 
B. Replacement of the leucokratic and melanokratic ele- 
ments one by the other, 
1. Melanokratic type, — Peridotyte. 
2. Leucokratic type. — Plagioclasyte, 
Modifications due to contact metamorphism, 
Disrestion of aluminous sediments. — Cordierite 
III, 
I, 
iioryte. 
2. Digestion of siliceous schists or quartzytes. — Quartz 
gabbro. 
3. Digestion of a potassic rock. — Orthoclase gabbro. 
Finally the silicofcrrolyte seems to represent a sedimentary 
rock entirely re-crystallized under the exomorphic actions of 
the gabbro rather than an integral part of the latter. The sili- 
cofcrrolyte differs from the gabbro from every point of view: 
it is much more dense, with a very dark color. After mag- 
netite, the mineral by far the most abundant is fayalite which 
occurs in large, irregular masses, enclosing large grains of 
magnetite, and being itself surrounded partially or wholly by 
pyroxene. Many rounded grains of quartz are enclosed by 
all these minerals (more rarely by the magnetite). Thus the 
texture has changed from granitic to poikilitic, though the fay- 
alite masses are mutually xenomorphic, 
CoMPA.RATiVE MINERALOGY. The minerals which occur in 
the rock series here studied are the following: 
Quartz 
Orthoclase 
Andesine 
Labradorite 
Cordierite 
Muscovite (Sericite) 
Biotite (Anomite) 
Enstatite 
Bronzite 
Diopside (Pigeonite) 
Augite 
Pectolite 
Anthophyllite 
Actinolite 
Tremolite 
Hornblende 
Chrysolite 
Fayalite 
Epidote 
AUanite 
Penninite 
Clinochlore 
Antigorite 
Bowlingite 
Apatite 
Titanite 
Zircon 
Spinel 
Magnetite 
Hercynite 
Hematite 
Gothite 
Ilmenite 
Limonite 
Pyrite 
Rutile 
Staurolite 
Graphite 
Calcite 
Mesolite 
Scolecite 
Pseudomesolite 
And one or two unknown minerals. 
